I just checked info about the Gamecube island... That was a mere 2 acres (1x2).
I guess I could see the appeal of a smaller island, given that its more a tiny resort than a full fledged town. I also think the actual dimensions made the Gamecube island more appealing: A more organic shape than a square/circle like the others. I think a 15 acre island (3x5) would make a similar shape, and have a good amount of room to decorate at the same time.