I do actually prefer the size of the forum as it is. I am using a high resolution on widescreen monitor. I have created a couple of message boards in the past that resize to the size of the window and this results in text being sprawled across all 22 inches of the screen, which I don't think is nice to look at.
The width as it is currently displays is what could approximate the size of an A4 sheet of paper and thus, to me, is much easier on the eye. The paragraph sizes feel 'right' as they much less often reduce to the size of one line right across the screen. This looks horrible when reading essays or writings.
I can understand if one was posting artwork or the like - then bigger could be better, but as the mainstay of the board is text, the sizing to me is just fine.
Just my two penneth.