If you look at Bizer.com you'll find that in 2006 Bizer calculated the number of islands to be 253 of which 132 were deemed to be inhabitable (i.e. >1/4 acre and hence potentially able to support a septic system). Only 73 are 2 acres or more. There's a lot of detail on the site about the research behind those numbers. Might be worth a read.
http://www.bizer.com/winislan.htm