[Silvuh]

942 pixels. That is how wide, right now, a background image is on a profile. If you upload an image wider than 960 pixels, it will be resized to fit that. The 18 pixels are cut off on the right.

Profiles can span 1500–3000 pixels (those are just arbitrary numbers) in height depending on how much information you have on it. (Tokens, long random thoughts or pictures in comments, &c.) Some people can get away with a 2000-pixel background. I had to stretch my height to 3000 because of my two lengthy random thoughts.

You can screenshot and paste and align your profile into an image editor, then crop the image to the profile edges, add some height for some wiggle room, and draw your profile on that. If you have a screenshot of your profile in the image editor, it's much easier to see where everything goes. But things like the "i'm online" icon and the size of your avatar will move information around.