What Bob said.
I'd strongly encourage you to sign up for the Aeroelectric List email list, and get the Aeroelectric Connection book. You can buy a hard copy, or download for free.
Read the book before ever running a wire.
There's a lot more that goes into sizing wires than the diameter of the connector. If you're just starting out wiring a plane, please do some research. If you don't, you could at best cause yourself a lot of grief, and potentially risk your life.
Sign up on the Matronics Aeroelectric Connection email list & gain access to a *lot* more knowledge and experience than you'll ever see here, when it comes to electrical/electronic issues in a/c.
edit: Should have pointed out, the issue isn't charging current. You could probably 'get away with' using 10awg for charging; that wire is typically quite short if the battery is on the firewall. But starting will draw *hundreds* of amps. You might not get enough voltage at the starter to spin the engine, but you'll almost certainly get enough current to melt the wire.