As it turned out, it was incredibly challenging to get the dimensions of the page and columns correct. I searched Google for this information but the dimensions that others provided did not work, and I could not find these guidelines on Twitter.com. I had to use trial and error to get it right, and ended up spending a couple of hours on what should have ideally taken a few minutes. So, in case anyone else would like to create a custom background, I decided to post the dimensions for the Twitter profile page.
Based on a display resolution of 1280 x 800, the canvas size of the Twitter page is 1265px wide and 700 px high.
Within the page, there are essentially three columns created by the "main" Twitter box, which lies in the center of the page and includes the sidebar which displays your bio. Here are the dimensions of these three columns:
Left edge of page to Left edge of Twitter box: 230 px
Width of Twitter box: 790 px (230+790=1020 px)
Right edge of Twitter box to Right edge of window: 245 px (1020+245=1265px=total page width)
Hope this helps!
