that depends on how big your team is, how much land vs. water your islands will have, and how much content density your game needs (i.e. survival games often have a lot of empty space while RPGs need much more stuff populating the world)
consider the land of Skyrim where the heightmap is 6.8 x 5.4 km
from that, only around 5 x 3.5 km are playable. and still it has an enormous amount of content in that amount of land