Long plugs are really only used for four stroke engines. Maybe some two strokes for giant scale use them, I'm not too sure. For most two strokes, you'll always be using a short plug.

I recommend the OS A3 plug for most two strokes. It's a hot plug. From personal experience, I've found the engine runs better with the hotter plug. For most flying, you will not need a cool plug since most glow planes should only be run on 5-15% nitro, unless otherwise specified.