There are four servo sizes which are tyically used on R/C helicopters: 

Standard:

These servos are usually 40mm x 20mm x 40mm, and are typically used on helis which heavier than 2.5 kgs, such as 600-700 class electric helicopters or .50 - .90 nitro helicopters, for example Align T-Rex 700, Outrage Fusion 50, Hirobo Freya 90 etc.

Some example servos include:

 

Mini:

These servos are about 30mm x 12mm x 30mm, ypically used on helis between 1kg and 2.5kgs, such as 500-600 class helicopters, for example MSH Protos, Align T-Rex 500, ...

Some example servos include:

 

Micro:

These servos are about 23mm x 12mm x 24mm, typically used on helis between 500 and 800 grams, such as 250-450 class helicopters, for example Align T-rex 450, Hirobo Embla 450, MSH Mini Protos, Align T-Rex 250, GAUI X2

Some example servos include:

 

Submicro / nano:

Vary in size, generaly speaking anything smaller than micro. These servos are typically used for helis under 400 grams, such as the Piccolo, GWS Dragonfly, Century Hummingbird, etc.

Some example servos include: