Short glow plugs are for smaller engines with less clearance between the cylinder and the head. I'm not sure where the cutoff for a long plug is, but it's smaller than a .40 so you want long plugs.

OS8 is a good standard. You can use a hotter plug for cold weather, for fuel with little to no nitro, or if you want to improve your transition and don't spend a lot of time at wide open throttle.

Colder plugs if you're using higher nitro content, if it's really hot or your engine is running hot, or for larger engines. My 1.20 comes with OS cold plugs. Large engines have more material to hold heat and thus do not need as hot of a plug.