What are some general rules for the mesh size and generators?

Yann_Le_Floch

Mesh size and density is essentially a trade-off between computation time and accuracy of the results. A higher density will yield more accurate results, but will increase overall computation time. As a compromise, the mesh density should be greater where there will be the greater variation in whatever measured value you are examining (electrical, magnetic, thermal, etc) (see below). There are also restrictions on the mesh size around moving air gaps.