You can use the 'Inspect Element' and layout features to figure out how the dimensions are being set.
There could be some very subtle CSS collision.
Other options would be to put some style tags after all the CSS, setting margins, borders, padding, and background - which will let you see the dimensions, then you can backtrack through the CSS until you see what's happening.
If it was a simple problem, I'm sure you would have resolved it by now. These are brute force ways to pursue it.