I would think that either a 15 or 16 guage would be adequate for doing
trim. I have used a 16 gauge for baseboards, and I actually use an 18
guage for 1/4 round trim, as the trim was stained and the nail head
less visible.

Come to think of it, I think I used the 18 guage for some baseboards
too, I can't recall exactly

So, if you're just doing baseboards, I'd try the 18 guage you already
have.