It depends on the bantam, some are a lot smaller and some are a bit bigger. The genral rule for cooking is 3 bantam eggs = 2 standard eggs. Here's a picture.
The light brown eggs are from standard SL wyandottes, the green egg is from and americauna and the small white egg is from my bantam japanese hen.
I have a couple of what I think are OEGBs, given to me by my mechanic last year when they blew into his yard during a storm. I think they are hens, as they still haven't developed any rooster-y traits and aren't crowing. But they haven't started laying yet either. I do wonder how tiny their eggs will be. They are such small birdies! Like large pigions!
I don't know if anyone still checks this thread, but I found it in searching for info on the size of bantam eggs. I have a silkie bantam chicken that we thought was a rooster based on behavior. Then I find this smaller egg today with blood on it, that I'm assuming is someones first egg? (The middle egg is a standard store bought egg for comparison). Besides my silkie I have Americaunas, Barred Rocks, and Wyandottes. Is this a bantam egg or did someone just lay a smaller egg?