There's only one thing standard with plugs and that's the thread size (1/4x32 UNEF). Long plugs can vary a little in length, depending on the brand, by maybe a thread or so. But both OS and Enya plugs are somewhere in the middle of long and short so call them a medium length. The ideal length is the one where the plug comes just flush with the combustion chamber but it's not really critical for normal use especially with Schneurle (flat top piston) engines. Small baffled piston engines are a different story.

Idle bar plugs generally aren't necessary although some people find they help with idle in inverted engines. Some plugs have the idle bar welded to the body and these have been known to break off...not good. The better ones have the idle bar machined as part of the body.

Then we come to heat ranges which is where things get a bit tricky. Basically the heat range sets the ignition point much like the distributor in a car. A hot plug fires it earlier than a cold plug but what you need depends mainly on the kind of fuel you use. The more nitro the colder the plug should be. If you've got a tacho then the plug giving the highest revs is close to perfect and this could even be a 4 stroke plug. But without a tacho and for general flying without too much nitro the hotter plugs like an OS 8 or Enya#3 are a good place to start.

To get some idea of the affect different heat range plugs have on how an engine runs go to
http://www.modeltechnics.com/glowplug/firepower.html