Not sure where you’re getting 250kwh/m2/year from. If it was one contiguous solid panel maybe you could achieve that and then you’d be correct it would be about 560,000 km2. Or roughly the size of France.
But you need to leave space between the panels in a solar farm for them to be at the optimal angle without casting shadows on each other. Real world solar farms have much lower density than that.
The density can vary significantly, our hypothetical solar island could be anywhere from the 6th to the 50th largest country but regardless we’re still talking about something in the area of a trillion individual solar panels.
Assuming money isn’t the limiting factor (which it isn’t in most countries) we don’t have anywhere close to the ability to manufacture and deploy that many panels by 2030 or 2035.
Assuming we maintain exponential growth of both wind and solar (doubtful) we’re still a least two decades away from eliminating fossil fuel electricity generation never mind meeting the 2-3x generation capacity needed to transition transportation and other consumers of fossil fuels over to electricity.
Renewables growth has shattered estimates before, you never know, but the transition is not happening any where near as fast as people seem to think.