1. People tend to lie about how big a guy/girl is, so asking for a size is just a way to get closer to truth (by just adding a size or 2). And by all means, if a girl is really interested or cares in how big a guy is, she should be able ask what size pants a guy wears and get a truthful answer.
2. A girl should also be able to see a pic before hand too, especially if one of the parties will be travelling.