But here's a slightly different perspective, based on being found in search engines and the relative value to potential users. Say if it's really valuable, quality directory of resources that fills people's needs. If each page has value, then they should be listed in the engines. If it's a fairly new directory it might not have too many inbound links or much PR yet, and a limited amount can only support a given number of pages on the site. Page one of a category may be indexed, but pages 2 and 3 might not and those might have equal value.
If it's a good site it'll grow in link strength and popularity, and IMHO at that point additional pages can be successfully indexed. Too many entries is too cumbersome for users, and too few gives them too much clicking, and it isn't pleasant to see a second or third page in a directory category (or a sub-category) with only 2, 3 or 4 entries.
How about a rough guess? Aim for 30-40-50 each max, and when there are 50-60 or so, subdivide to add a sub-cat. It's better to have a few more on a page for a while than to subdivide so there are empty categories with nothing but global elements and navigation. Those are what Adam Lasnik from Google has referred to as stub pages, and those aren't search engine or user-friendly.