I set mine at 50 mb for my clients, for a variety of reasons. A lot of them will be wanting to run PostNuke, which alone takes ~5 mb. Add a few other scripts and a good chunk of space is gone. It really depends on how you are promoting your site and what you are focusing on. If you are really hyping PHP capabilities, then giving someone a 10 mb site could result in some very unhappy clients. =)
I know the vast majority of them won't even come close to the 50 mb - which is perfectly fine as it gives me more room to play with when it comes to my sites.
Hope this helps,
Wen (whose main personal site is about 250 mb so maybe she isn't a good person to ask about average web site size...