What breaker size and wire gauge should I use for a new garage outlet circuit?
You didn't mention what type of equipment you'll be using, nor did you mention if it was an attached or detached garage.  Without a bit more detail, it's not possible to give an accurate answer. As a general guideline, you'd use 12 AWG copper conductors with a 20 ampere breaker, and 14 AWG copper conductors with a 15 ampere breaker.  Since the receptacles are in a garage, they'll have to be GFCI protected. This protection can be provided by a GFCI receptacle, or a GFCI breaker.

If you're going to be using multiple pieces of equipment that draw a lot of power, you might want to consider running a 20 ampere multi-wire branch circuit. If you have any locations where you'd use multiple high draw items together (e.g. saw and dust collection), you could install a double gang box with two duplex receptacles. Feeding each receptacle with a separate leg of the circuit, will allow you to use two high draw items without tripping the breaker. 

Since it's a garage, I'd install a single 20 ampere circuit at the very least. A 15 ampere circuit would be fine for lighting, but you'll find it quite limiting when/if you start plugging in tools/equipment.