A rule is, if you have a 50x50 exhaust fan then make a 50x50 hole but like I said before, it will depend on your exhaust fan sucking capability.
For passive intakes, I'd make the combined area 25% to 50% bigger than the exhaust fan. The greatest obstruction to air flow is at intake and exhaust, so if one of them is unpowered we can just make it bigger to reduce resistance. This is my experience from cooling polytunnels (hence rabbits, lol).