Ok, it's a big island. My thought is that there would be a mtn chain in the north and some mesa things in the south. Unless this is a southern hemisphere island then flip that. The reason for that is purely aesthetics...mtns are better in cold areas and mesas are better in hot places. Since this isn't a continent there probably won't be any cold places though...islands, even big ones, are pretty static and the temp won't vary all that much. So if this were in a temperate climate it would probably be grasslands and forests with some cold areas at very high elevations. Really high elevations would require quite an extensive set of mtns but you could probably get by on this point. Extensive mtns would also let you have a desert. You could probably get a swamp or two in as well. If this were in a subtropical climate it would mostly be desert and grassland and mesas. Tropical climate would have a lot of jungles and swamps and no desert.
Now someone else will have different ideas and since I'm not a climatologist these are open to debate but this is to the best of my knowledge. Here are two ideas.