You can walk across it in a few hours, so Hawaii size? -or- It's big enough to have several distinct climates, and regions, so continent size?
Translating a game world into real-world size references is not easy and would require suspending strict adherence to physics, in-game content or reality. That's just the nature of designing game worlds.
Scattered in the game, there are a few references to how long it takes to travel in the Kingdom. Some include statements from River boats captains, or the NPC's in teh quest Those in Need. The first say it takes only a few hours to travel by boat between cities, the latter said it took weeks to cross the desert. You do both in a minute or two of walking. So not consistent size references.
Personally, I settle on the Island being the size of the North America Continent with a vaguely similar shape. Keep in mind, that while there are obvious similarities between Varannar and real-world North America, it would be presumptions to draw too much of a connection there. North America has neither dragons, magic.