I want them to be 1:1 scale. So, Isla Nublar being the same size as in the movies for example. If we go by a map like this from Wikipedia (though I'm not sure where they got this from), then most of the islands are larger than Nublar, with only one being smaller. I've not bothered to actually look into the actual sizes of any of these islands though, so I don't know if a 1:1 scale is reasonable.
Personally, I hope for custom maps as well, but really what's important is the Park customization, not the island customization.