Every kit or ARF I have seen lists a range of recommended engines, usually a 2 stroke range and a 4 stroke range. Some people like to use the minimum recommended size and fly their models similar to the way many full scale planes fly (even if their model is not a scale model). Others like to use the maximum recommended size (or bigger) and fly their planes in ways most full scale planes could never fly. Neither preference is right or wrong, you should just do what is fun for you.

A lot of people pick the plane they want and then choose an engine that is appropriate, based on their preferences.

My preference in airplane size is models with wing span between 55 and 72 inches. This usually makes them "40 size" or "60 size."
My 40 size models get .70 four strokes (maximum 4S recommendation for a lot of 40 size models) and my 60 size get .91 four strokes (except one that has a .61 two stroke).