Please what is the average legion size?

Not sure what an average size would be. As you level up players will have different numbers of troops in their various legions. Once you max the hero skills and training ground levels the legions can reach 325k troops.

Size is not everything in many situations. Good hero skills will let you defeat other players with larger forces although it will depend on the tier of troops and matching the right heroes to your troops and situation.

Some activities do benefit from having the highest number of the highest tier of troops you can manage though.