everyone has 30x30 cages at my place, but bred does go into 30x36 cages so they have room for them + a nest box and the subsequent litter until they're weaning age. once their litter is moved the does will go back to their regular cage. i have standard rex, mini lops, and meat mutts, and i feel like anything smaller than 30x30 would be too small. in these cages they have plenty of room to lay down and stretch and hop around a bit. i also regularly let them out into a big 8'x8' covered play yard to run around and munch on grass. they seem much happier and in better condition when they get to run around.
i don't breed in cages though - the does always seem to panic because while 30x30 is enough room for one rabbit to stretch out and be comfortable, it's not enough room for two to run around and do the breeding dance. they much prefer to breed on the ground. the play yard seems to be too big (the does run too much and he can't catch them haha,) so i have two expens i set up that gives them enough room for the doe to not feel trapped, but not too much that he can't catch her when she plays hard to get.