Ok, thanks for all the help. I think I have figured out how to do it. Here is the process I used. I took a map I had made using the Saderan tutorial that one of the land layer masks on it and used that to make a selection of all the land. Then I used quick mask to get rid of all the islands I didn't want. I made a new layer and filled the selection of the island I wanted with white. Then I made a new document with the proportions I wanted. I went back the the original map, clicked duplicate layer and changed the document to the new map I wanted to make. Then I used free transform to increase the size of the island about 950% since 1000% wouldn't fit. The edges look a bit blurry after this so I apply a smart sharpen filter at 500% and about 12 px. It gives a good result.