Somebody asked it here, and the answer was basically "there's no specific size". Still, I'd like to ask if there's an average size to it. Would a Greenland-sized landmass cause elephants to evolve into smaller sizes? (On a side note, were there mammoths in Greenland?)
I don't know the answer properly, so I welcome other input.
You can probably work this out more or less empirically. Going through the (possibly incomplete??) lists of island gigantics and insular dwarfs you can see a list of which islands they have been found on. Also ignoring the dinosaurs, as we don't properly know the size of islands in those cases
The largest listed is Madagascar and it's about 1/4 (actually 0.275) the size of Greenland at about 587,041 km2 / 226,658 mi2 . It may be that this is about the limit.
Note that this answer relies entirely on the current sizes of the islands, it does NOT take into account previous changes in land area from such things as sea levels raising/lowering. I am almost certain that there will be more to it.