The wire size will be determined by what the loads will be in the garage. If you just need a few receps, lights, and door openers, 30a should be plenty. If you're going to be like Norm Abrams, maybe 50 or 60. Very few people truly need 100 amps in a garage.
You can feed it with whatever size breaker you want, up to 100. 15 amps if that's all you need. Just size the wire according to the load expected.