I can not answer your question for predicting the size of robber camps, but yes it is based on number of troops you own, size and age of server. I would not overly concern yourself on predicting its size because basically
The more successful you are the stronger your robber camps are
there is no way of avoiding this
To help answer your question on how to deal with robber camps as a def player, Gaul has it the easiest.
You can let the robber camp hit your traps before you attack the robber then clear the robber camp and the troops will magically disappear from your traps
Another advantage Gaul have is they can build Hauds which are a duel role troop for clearing robber camps then serving as a defensive troop when the time comes as well