Wiring a 2 pole circuit
Each circuit has a minimum wire size.  

Other than that, you can upsize wires anytime you want.   There's no restriction.   

The wires must, of course, fit the terminations.  So if you use #12 Romex to branch off a 15A circuit, you will not be using backstsbs on that receptacle.  

To reverse it, if your circuit is almost entirely #12, but has one bit of #14, it is a 15A circuit.