Does a practice (or firm) have to be a certain size before it can be editorially recommended?

No. However, the size of the team, and its strength and depth, are among the factors which the editorial researcher will take into account when assessing a practice against its competitors. Others include the quality of work handled, the calibre of clients, referee feedback, and the reputation of individual practitioners. But a small department can still be worthy of recommendation in The Legal 500.