I know that a 20 amp breaker and a GFCI plug are required in a bathroom. Does this mean the wire size has to be 12 gauge or can it be 14? If I wire the bathroom lights on the same circuit, can I splice in and use 14 gauge for the lights on a 12 gauge feed?

Only #12 wire can be used on this entire 20 amp circuit, cannot use any #14 at all.

That's what I was afraid of. It wouldn't be a big deal but man copper is expensive right now! I have lots of 14 gauge, but no 12 gauge... go figure. ;)