[Gundam-Dude]

Pretty much any dimensions proportionate to 256x80 will work as long as it's larger in size. 512x160 is just easier to remember because it's doubled, lol. I've tried banners of 400x125 and they also work exceptionally well, however it just -looks- strange since people are used to seeing the former two dimensions.

Also, SM5/SSC supports widescreen ratios so you could also take that into account when making banners and backgrounds for files.