background advert specs

hi – please could you advise where i can find out the specification/dimensions i need to give to an advertiser if we want to use the ‘background click ad’ option for the site? i can see where you put in the url for the click through, and i can see where it says upload an image in the background tab to use for the advert, but it doesn’t specify the size of the image that should be uploaded, and when i upload one it obviously appears shrunk up at the top right hand corner. it tells me to disable the ‘stretch background image’ option if i am using a click ad…so how do i find out how to design an advert to suit this background area (i.e. so the right information in the advert appears on the sides of the site)?

thanks

thanks, obviously with the site blocking the centre of the advert/background the advertiser will need to put his sales message into the only visible parts – the narrow far left and far right columns…so my remaining questions are:
1) is there a way of customising the size of the actual website page (e.g. making our webste page slightly narrower? not sure how much coding/customisation this would take? ) to give the advertiser a bit more space?
2) do you know of any other website using the newspaper theme that has got background advertising in place? perhaps i could contact them to see what specs they used, or at least view it to see how it can work?
we just need to present to the advertiser a professional spec or example, so they can simply provide us with the right artwork/image to work…
any further assistance you can provide would be greatly appreciated.
thanks!