Yep, depends on the size of the birds (and their feet) but about 2x2 or a bit less. Take off the corners a bit too so they're a little rounded - no sharp corners. Height is tricky. I've got big, heavy birds so they won't perch if the perches are too high (and can injure themselves getting down). But if you put it too low, they might end up in the nestboxes instead, making a mess of them. I believe it can be good to have hanging perches so you suspend them from the roof with chains. It makes the birds work a bit harder to perch which keeps them fitter and might be easier for you to put in.
H