I looked around the forum for you, but all I came up with was this from the FAQ:
How big should my nest boxes be?
For larger breeds, a box 12" high, 12" deep and 14" high is fine.
Mine are smaller than that, but they lay the eggs in them. So for the bantum, I guess proportion it down. Good luck.
I don't know the minimum size for a bantam nesting box. It is a good bit of information that should be available on this forum, but I don't remember ever seeing it. Why don't you build the nesting boxes the size mentioned above? Bantams will lay in it and if you ever decide to get full sized fowl, you are prepared. Sometimes they like to crowd in and lay together. If the nesting box is larger than the minimum, they can do that comfortably and maybe with less chance of stepping on and breaking each others eggs. Odds are they will all want to lay in the same nest anyway. I made mine 16" x 16" for full sized fowl and it is working OK.
I don't know your goals, plans, or intended management techniques, but I'll show a photo of how I built mine. The two on the end are the nesting boxes. The area in the middle can be accessed from the back. If I get a broody that I want to hatch eggs, I can block the nest off from the other hens, open up the middle for the broody, and put food and water in the middle section. That way, I don't have to move a broody. The middle section can be used as a broody buster. With bantams, you will probably need that. If you need to isolate an injured chicken, you already have the space. I raise chickens for meat and eggs, so I have this space to isolate the chicken with water but without food for a day before I process it so its digestive system is clear. It's really only big enough for one chicken at a time, but I have found it to be very handy. Just something you might want to consider.