What kinds of animals does insular dwarfism affect and how large does an island need to be to escape it?
There are no general rules. We have just observed repetitively that organisms living on small pieces of lands tend to be particularly large (insular gigantism) or particularly small (insular dwarfism) compared to the main land counterparts.

There are several possible reasons for these changes and those reasons vary among species. Dwarfism is most definitely not always a consequence and there is no hard coded minimal island size for such change to happen or not happen.