Bathroom fan do I use 12-2/20a or 14-2/15a
Your fan really only draws a few watts. You need the wire size to match the circuit breaker if it is a 20 amp circuit breaker you must use 12 gauge wire. 14 gauge is too small and can start a fire.  If it is a 15 amp circuit breaker it needs to be 14 gauge or larger (you can use a larger wire gauge than required). 
The circuit breaker is protecting the circuit wiring. 
The UL listing says it’s OK to use in the US. On a 15 or 20 amp circuit.