What size roof speakers go in a 2000 Jeep Cherokee?

There's no single "size" roof speaker that fits all 2000 Jeep Cherokees. Jeep Cherokees didn't come standard with roof speakers. Adding them would require custom installation, and the size of the speakers you can fit depends entirely on where you decide to mount them and what modifications you're willing to make. You'd need to measure the space where you plan to mount them to determine the maximum speaker size that will physically fit.