My intention is to start with a single island for low/mid-level adventures, then add an island or two to the north for higher-level adventures. The northern islands will be frozen year-long (as they won't be warmed by ocean currents) and be infested with high-level monsters (white dragons, mammoths, wholly rhinos, rogue fire and water elementals and clockwork constructs); they'll be there for high-level adventures; a nearer and more reachable island will have hordes of snow orcs and a mid-range mage's tower. They'll be reachable either when the PCs buy or build a seaworthy ship (or steal such a ship from the snow-orcs for the nearer island) or gain Teleportation at level 9.
So up to, say, level 7-10, the game would be confined to one big island and a few smaller islands adjacent to it.