The in-universe solution to the Infinite Sea problem, crossing millions of kilometers of landless ocean, is actually "create a flying island of X acres and take the trip in style" because there is simply that much magic in the setting and it's that easy to do the "flying island" shtick. Also because wizards are the only people in that world with a real curiosity about what is over the horizon, and why would they risk sea sickness.
Given that it's easy to get such a flying island state into the air but impossible to provision it with food and craftsmen etc. using magic I wonder how big X needs to be to create a self-sustaining high medieval city-state that can supply all its own food and fuel, in the form of wood and its products. A limited supply of things like potters clay, building stone for repairs, and metals and/or their ores, can be brought in but for the rest the state is effectively cut off.
Assuming a late medieval agricultural environment, including arable crops, livestock, woodlots and hedgerows, and complete with occasional total crop failures, etc., in a mild, well-watered climate, on fertile soil how much land would an island state need to sustain the full social spectrum of a feudal hierarchy (so a near absolute ruler, a landed gentry, and the merchant/artisan and peasant majority).
Given this excellent answer I don't in fact need a land area so much as a population. My understanding is that single fiefs were never self-sufficient for much more than the bare necessities. Although they often supplied the raw materials for some luxuries to their feudal overlords which were processed in specialist workshops like wineries located close to the seat of the local Baron. Even single baronies seem to have needed to trade for many luxuries, some produced good wines, others fine cheeses etc., and they traded for the goods they did not themselves produce. So the question becomes one of how many people it takes to make a self-sufficient kingdom, albeit a small one?
The state will be effectively coastal for the purposes of a fishing fleet but the numbers given in Mołot's answer don't change much if at all because of this so it's still a matter of the population to sustain the crafts and land use diversity to supply the kingdom (wizarddom?) with everything it needs in the form of both the essentials and the luxuries of life.
Please note specifically that while the island itself is a magical construct the means of supporting the population on it cannot be.
In case it makes a difference the state will not need a standing army for the trip although the equipment for a militia levying all able-bodied adults will be maintained.
Special thanks to the people who helped me sandbox this, the original didn't really ask the right question.