I'm not a wiz in rezing so what I did is I took a banner of appropiate size from a user here, I opened it up at a Paint Shop Pro, and I pasted my banner on top of it and I just resized my banner image to fit the other banner...

There is setting with will let you input the appropiate dimensions, but since I'm not familiar with the appropiate dimensions I used this method and it worked for me fine so you could try that...

someone will probably give you a better way to do this...