sub panel in garage?
You need 4 wires for a subpanel (neutral and ground separated) - as such, 6AWG is all that fits - 4 AWG would require 1" conduit at a minimum (even for 3 wires.)

I don't keep this in my head - I use this, or one like it:

http://www.electrician2.com/calculators/rf_calculator.html