I assume you talking fence posts here and in which case, 3x3 is more than adequate. If you are lucky enough to be working on level ground and the pen is a reasonable rectangle. then possibly 4x4 at the corners. Have you thought of using electric netting? Comes in 50m rolls (with posts) and by the time you have included an energiser and battery (if not a mains energiser) the cost would be much the same. However, if you have a wild rabbit problem, don't even consider it.
Guinea fowl & a lot of surplus poultry equipment.