HI,
I want to install a ceiling fan. I can put a junction box into 14/2 wire. Can I splice in 12/2 wire to run to the fan? Or would the 14/2 be good enough? I'm just worried because there are other lights on the line.
Thanks,
D

If 14-2 is existing, you can stay with 14-2, or 12-3 which is larger and more expensive.
Still need to keep a 15 amp breaker for the smallest size wire.
If all is #12, you can install a 20 Amp Breaker.

I am not sure what you mean by "good enough".
If the rest of the circuit is wired with #14 than you should stay with #14, and that is certainly good enough.