The following are a couple things I always like to point out to my candidates if they express a general preference for a particular size of firm, depending on their reasons for doing so:
A firm may be a 3,000+ global mega firm with dozens of offices in various countries around the world, but just because a firm is big, the size of the office in the city where you are looking to relocate may not be, and that can have a dramatic impact on how it feels to work there relative to the stereotypical differences between big and small firms. A 20-person office at a large overall firm may have a much more intimate, personal, and collegial feel than the 75-person office that is the only office of a mid-size regional firm. Thus, if you are looking for a situation where you know everyone in an office and aren't just one of hundreds of attorneys, don't automatically pass over a "big" firm. Spend the few moments it takes to look at the size of the particular office in the location where you would be practicing.
Practice Group Size
Similarly to office size, the size of a given practice group within a particular office can have a large impact on both the "feel" of working at a firm, as well as the scope of opportunity available to you, which can be especially important as a more junior-level attorney. An office might have 200 attorneys, but if you are a patent prosecutor and the patent prosecution group in that office only has two partners and one other associate, guess what - you will almost always be working with those same three attorneys.
Along similar lines, it is helpful to research how large the overall practice group is at the firm as a whole, and whether assignments are always office-based, or are cross-collaborative in the sense that you might be working for partners in multiple offices. This can factor into the scope of opportunities you will be exposed to, usually the broader the better, but it can also mean that you have to be "on" and "available" for partners in multiple time zones if your particular office won't have enough direct work to keep you busy, which can dramatically impact your work schedule and thus your quality of life.
Another point to consider in this same category is how much you want to be the "go-to" attorney in your practice area. If you are the only attorney in an office with experience in international tax matters, then you will receive the majority of that work. This can be a double-edged sword. If you like that type of work, then it is great, and you also enjoy the added bonus of being more indispensable to the firm than if there were a half-dozen attorneys with your same skill set. On the other hand, you run the risk of being pigeon-holed into a particular niche practice area because everyone relies on you and you alone for that skillset. Whether a particular situation is good or bad in this regard is up to you and your long-term career goals, but either way it is worth thinking about.
Promotions and Firm Politics
If you are looking for a smaller, more collegial environment, it is very possible to find that at a large firm if you are working in a smaller office. The potential downside of being in a smaller office is that the firm's leadership/location of power is typically centered by nature in one of the firm's largest offices. The track to partnership is difficult, and involves many factors, but one indispensable factor is having a champion for the progression of your career in the form of a more senior partner on the partnership committee who is willing to go to bat for you. Even if you have such a relationship, that partner may not be as effective of an advocate if they do not interact as regularly with the general leadership, or do not have the same kind of rainmaking pull as a big partner in the big central office.
If you are hoping to eventually advance to partner, especially at a larger firm where the opportunities are few and your colleague competitors are many, it is very much worth considering how realistically you can develop the necessary relationships with enough senior partners, especially those who have pull on the partnership committee, something that may be more or less possible depending on the size of the office relative to the overall size of the firm.
A counterpoint to the prior section, where it might be better to be in a larger office than a smaller office, is how much potential you have at a given size of firm (and size of office) to develop your own book of business. In a larger office with a couple rainmaker partners who farm out their work to a pool of associates, you may be stuck working away in relative obscurity for years without getting the attention of a senior partner, or having much in the way of client interaction. But if you are at a smaller office where matters are more leanly staffed, you might have the opportunity for more high-level experience and direct client interaction earlier on in your career, both of which are extremely important to client development. In particular, if you are the sole mid-level associate of a particular office, and will thus always be staffed on matters for the partners in that office, you are more likely to be included in client pitches, business dinners, and local industry panels and presentations, all of which can represent solid exposure for you and your career.
Additionally, a lot of business at large firms is legacy business, so if you can position yourself well at a smaller office and become the right-hand of the right partner, you may stand a good chance of inheriting that business down the road.
In short, it is worth exploring during the interview process how promotions are handled, how much support and encouragement is given to associates in their business development efforts, and the track record of a particular office in terms of promoting associates to partner relative to the rest of the firm. Sometimes the bigger firm or bigger office is better, sometimes the smaller firm or smaller office is better, but it is a case-by-case basis where the general headcount is often uninformative at a glance.
Newness of the Office
Sometimes how big a firm is can matter a lot less than how new it is, and the same goes for a particular office within a firm. For instance, a number of larger, longstanding national and international firms have only begun to establish a west coast presence over the past decade or two. Getting in on the "ground floor" of a particular office can mean you immediately jump the line, so to speak, in terms of relative seniority, potential for promotion, and positions within the overall firm leadership. Particularly at the partner level, you may find yourself having more influence and more leeway to handle business as you see fit at the newer and smaller office of a large firm than the mid-size office of a regional firm that has been established for many decades.
The flipside of this is that newer and smaller offices can often feel like an afterthought to the firm leadership in terms of resources (and particularly hiring resources), as well as marketing and promotional support for that office's business development efforts. It is worth exploring in the interview process how resources are allocated to that office, how much discretion the office leadership has to run things as they see fit rather than be beholden to a far-away central hiring or management committee, and the track record of that office's expansion efforts, hiring, and retention.
As you can see from the above, there are many more moving parts and salient factors to whether a firm might offer what you are looking for rather than just going by general "firm size." Firms big and small can be collegial, or incredibly toxic, depending on the firm and the office. Business may be easier or harder to develop depending on many factors beyond just attorney headcount. And your work opportunities may be enhanced or limited, depending on whether you are looking for breadth of experience and the opportunity to work with a wide variety of partners and clients, or if you prefer to develop deep expertise in a particular niche area or establish a longstanding working relationship with a few select partners.
You should do your own research on all these factors, ideally before becoming too heavily invested in any particular set of firms, but definitely before making any decisions on joining a law firm that may impact your career in the long-term. And of course a good legal recruiter should be able to guide you through this process and have solid information on the factors listed above.
As always, best of luck with your law firm job search!