Might help to know what you mean by "massive." Technically almost every seed is a massive island if you went far enough to find the surrounding ocean... anyway, the seed I posted before, "6yh" is a nice big single-biome forest island.
Heres a massive island seed: -4017893226529467483
Its actually one massive, and one large island right next to each other. Only thing really lacking from it are Sheep and Chickens.
Also if you set it to Large Biomes world type, it'll instead generate to a few small islands, that are quite sparce (the nearest island doesn't have a tree, but a island to the east does.) so the seed can act as also a survival map in a way.