Why not just blow it up to the size you want at Kinkos or something? They might be able to size it right while maintaining image quality
I don't have a Kinkos in my town. I only have Wal-Mart photo lab, and the prices to do that is way to high to make a custom boxart. Couldn't someone just scan their boxart and post it here? There was one at Uesp that was bigger but it won't load on my computer.