The usual dimensions seem to be 770x650.
Sasuke BG
http://fc00.deviantart.net/fs71/f/2013/151/e/6/sasukebg_by_ogskips-d678svv.jpg
Sasuke BG (No Sharingan)
http://fc00.deviantart.net/fs71/f/2013/151/2/a/sasukebg_no_sharingan_by_ogskips-d6794bt.jpg
These are my first attempts at this so they aren't the greatest
Use these to your liking