What size of braker do I need if I have a size 10 wire for A/C

If you look on the information plate one the side of your condensing unit it should list the maximum current protection for your specific unit.

To elaborate on what mark said, the breaker needed and wire size required depends on how many Amps the unit is rated for (draws).

For Example 15 Amps requires at least # 14
20 AMP at least # 12
30 AMP at least # 10

So if you know you need size 10 wire odds are unit draws a max of 30 Amps. But like he said it should tell you on the unit how many Amps it requires. Wire accordingly.

For simplicity I am leaving out voltage drop considerations, but if the unit is a sizeable distance from the service panel you may need bigger wire. How far away is it?

Most condensing units will tell you what the min. ampacity rating is as well as the max. fuse protection size.