Size:Any size that fiats with the render and the size that you feel is good and a size that fits these forums.
Images:<- you'r going to have to render this one, if you can't use a smaller version of the picture which is rendered which is here:.
Other(Seriously, put anything you want to add. I bet I could do it. ^^):Please put a grid in the bacground if you can, scan lines(white or black, the one you prefer, try both and see which is better) and put a border like this: