Ebay do NOT request that images be a minimum of 1600 pixels. They recommend it but it's not a requirement. The minimum required is 500 x 500.

If they required 1600 I'd have to give up selling because my computer would never load pictures that big. Also, I scan them so they show at actual size on the screen (my newer pictures) and they come out at just over 1,000 pixels on the longest size. If I did them any larger people wouldn't see them as they really are, and there's nothing like seeing things at actual size to know if you like them.

If I edit them from A4 size to square so that they fill the gallery picture space they won't enlarge, so they probably don't enlarge until they're at least 1,000 on one side. I'm not sure on this though, even though I've experimented a bit.