Well, I haven't had any experience with the gentlemen yet, but I do have enough experience with toys to know my comfort zone.
Since my vaginal canal is on the short side, a guy will have to either be 4-5" long at the most or accept the fact that he's not going to fit all the way without banging my cervix. I hear about ladies preferring 6-7" or longer and I cringe. At least my toys won't ever be disappointed if they're a bit too long to fit all the way.
As for girth, a diameter between 1.25" and 1.5" is preferable, but I'd be willing to work up if a guy was patient with me, or down if he didn't mind experimenting (though of course, a willingness to experiment would be good either way).