Hi!
First of all there are 3 sizes of glow plugs! Long, short and the japanese OS and Enya plugs which is in between long and short!
Second.
There is no rule that smaller engines like a .10 or .15 (1,8cc and 2,5cc) should use short plugs!!! For instance all small 1,8 and 2,5cc engines have always been supplied with the OS 8 plug... which works perfectly good for these engines, and most other two stroke engines too.
You can also use long plugs like the Nova Rossi 3-4 or Rossi 3-4 in these engines if you like.

It is important that you choose the right plug but the right plug is more so linked to what fuel you use and not only to what engine you have.
For instance all two stroke engines function perfect on 0-15% nitro and the there is a wide variety of plugs that works well, like the Enya 3 or OS 8 for most sport engines.
But for out most power, in a .40 engine, using a 10x6 or 9x7 APC or RAM prop....like in Q-500 pylon racing running 0-5% nitro; Nova Rossi 4A-5 or Rossi 3-5 are hard to beat. But of course there are others that work good too.