I agree with Fable and BS..
I would prefer to keep my avatar if possible, and as BS notes, being forced to choose from a pre-selected set of avatars tends to irritate forum members generally. I know I've been part of other communities where this precise issue has been a major point of contention.
I'd be happy to make my Ava a standard size if that would help?
Definitely, I do think it would make sense to have new members choose from a pool of avatars, for the reasons you state, but my feeling is that it would be better to allow customization later on.
Maybe it's even something that could be granted as a perk, along with gaining exalted member status? I realise this could lead to mindless spamming.. but if not abused it could also be fun, and it would give more meaning to becoming an "exalted member."
One thing to bear in mind, from a marketing angle as well, is that features such as avatars, signatures and locations provide individuality in an otherwise anonymous internet setting. Taking these choices away from people tends to cause a lot of upset.
Though, clearly, it's your site and your decision in the end, Buck