The length of the plug is dictated by the engine. Nowadays, all but the small ones like .15s and under take regular length.

Actually, you choose the plug heat range to match the amount of nitro you plan to use. THEN you test fly to decide if that's working for you.
Only the very high nitro events usually use special plugs and then it a cool one. (not what you thought, right)

The insider info on plugs is that there really aren't any magic ones.

Some people suggest using a 4cycle in 2cycle engines. If you think that $8 plug does better than a $4 one the mfg suggests, have at it. If you think something is helping you, it actually is. Even if it isn't helping.