The skin "takes" 1024, but the originals were simply made in 256 (hell I think I mighta seen one at 128 -- can't be sure). To make skins for these planes requires you to put a bit more effort into it, and start on a blown-up default skin so that you're actually working at 1024. Then proceed from there, making things at the proper size.
Takes more work than the new skins, but some already have templates out there (f4u's included) at 1024 res.