CookieRevised is right, and wrong at the same time. Messenger CAN read .jpg, .png. etc as the background, but the file extension needs to be .gif for it to load.
It doesn't matter if the image is a .png, RENAMED as a .gif, or if the image is a real, true .gif.
BUT messenger can't load a .png (with a .png extension) as the background image.
And as for finding the contact list size, I'd suggest making it the size you want, print screening it, and bringing the print screen into photoshop or something. Then crop out the contact list, and look at the image dimensions. This will give you the contact list size. Altho, making the window bigger/smaller WILL alter your image
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EDIT: Oh crap, both answers I just posted were already answered! Oh well, I guess another explanation doesn't hurt
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