The short plugs are for .15 size engines and smaller. The long are for anything over that. A short will work in a large engine, at reduced compression in a pinch. A long plug in a small engine will sometimes hit the top of the piston. Idle bar plugs were designed for cross flow engines to keep the element from cooling when the bypass fresh mixture came in. These days with schnuerle porting on just about every engine, they are not really needed although for many it is a hard habit to break. As to hot or cold, If you remove power to the glow plug and notice a drop in rpm, you either need a hotter plug or more nitro. If no drop is noticed, you are probably right on. If on the other hand, you hear a frying egg type noise, your plug is too hot or you are running too much nitro. Hope this helps. To be quite honest, most engines aren't too picky!

Max