You'll get a feel for it
I usually use 2 and a half inch 18 gauge nails to install doors, crown, and base. If you nail into the meatier part of the wood, at least a half inch away from the edge or edge of design, you shouldn't get too much splitting. If you hit the top or bottom plate, or the stud, 18 gauge will hold strong. Though I don't usually use a regular nailer for the small edge of casing that gets nailed to the jamb; for this I use either a pin (baby) brad nailer, or a small crown nailer.
I never or rarely use anything bigger than a pin or baby nailer to install shoe, toekick, cabinet crown, or any other fine small piece of trim. I also use a pin nailer to secure outside corners of moldings including base, crown, and small pieces of base.
I glue all outside corner joints (except stain grade wood).
A small crown stapler comes in handy for odd jobs, like for instance, a thin piece of wood a regular nailer would just shoot the nails through or a stubborn piece of casing that won't hold with a pin nail.
I have a porter cable angle finish nailer that uses the bigger size nails, but I rarely use it unless the other guns need service; I've never really thought of it till now, but I guess it's because the gun is heavier, and the nails cost more.