Mark from Smashwords
blogged about cover sizes a bit ago. Since we have to deal with almost all cover size requirements with various retailers, he does a solid wrapup.
Apple requires the shorter side (the width) to be at least 1400px. I'd *personally* suggest that when you ask for the art from your cover artist, you request it be at least double that (2800px short side) and then scale down when you upload if you want your cover art to be future proof. The new tablets and retina screens will soon be wanting bigger covers from us.
Final note; the higher quality and resolution of your cover, the more likely it is that Apple will pick your book when they select books to feature.