WHAT IS ELEMENT SIZE IN FINITE ELEMENT METHOD?
Easiest way without going too deep is to do multiple simulations of same model with various element sizes and analyse/ compare the results. When the result value and mesh size is ploted as XY graph there should be function line that with large mesh goes in total Giberish but when mesh size is reduced it converges/ tends to some single result. From this the mesh size for model to get ~ "reliable" result can be gained. mesh size needs to be in range where graph is flat.