650px wide thumbnails

I have a site selling signs and wish to add printed PVC banners to it.

Obviously for people buying safety signs 150 px wide thumb is fine but the banners need to be at least 600 wide so they can be seen on the list pages aswell as product detail.

I have done a test product to see if it will take the banner images @650x200 which it does and would be ideal but the sign products that are already on the site get increased to this size also. Which takes up too much space on the sign pages. banner, CAn I get a nice big image of the banners on the list and detail pages without affecting other thumbs.