What is "size"?

[E39M5]

I am a newbie and may have the most silly question of all.
Would someone tell me what are the sizes of helicopters?
30-50, 60-90, etc.
What do these number mean? Are they for size of the bird or the engine?
Thank you.

[Fasrc]

That is the size of the engine the fits the heli. For example a .50 engine mounting ship is considered as a 50 class heli. ;)

[Motions]

To explain a little better, a 50 size heli uses a ~50 size engine. Engines are measured in cubic inches (c.i.) so a 50 size engine is .50 c.i. or half a cubic inch. The bigger the heli, the bigger the engine has to be.