Probably depends a bit on the max size of the clowns as well...I'm guessing you're going for Picasso's, so the 29 is probably okay but I'm with skimerwhisperer, more space is always better
Seems I recall reading that many animals are more likely to breed in stable environments(larger usually=more stable parameters) and those which more closely resemble their natural home(i.e., large
...in fact, I have a nice almost-empty 55 gallon they could have...of course they'd have to live at my house since that's where it is... JK :lol:
29 would probably be ok, I think there were a couple guys on RC that use 29g cubes for breeding clowns. I would use what you have/is easiest but I'd lean towards the 40b.