Hi,
Actually, it is possible. I did it on my sophy template based site. I changed it to 750x225 or something like that.
I can't remember where I changed it, but it might have been in the stylesheet or in the admin includes. It was one of the first things I modified, and I remember it because I didn't use the template overides - didn't know what I was doing, like most of us when we start.
May I suggest that you use the search tool (tools, developers tool kit) to search for the 750 and you will find the location to make your changes.
After that I had to muck around with padding in the style sheet. search my posts and you might find some further answers on this... it was awhile back. Usually I will post solutions to my own problems if I find them myself.
Good luck!
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