I think there should be a guideline for everyone to follow with regards to dimensions and possibly background colour set by photon and Bingo. There's no point in voting on an image that only looks good at 400x400px on a yellow background if it will only ever be used at 100x100px on a red background.
The banner ad at the top of the page is 728px wide. With the current logo and the banner ads, plus the padding around them, I have to have my Chrome window (on Windows 7) at a minimum of 1042px to not get a horizontal scroll bar. That's with the current logo that is 239px wide. If the new logo were 500px wide, then the new minimum width becomes over 1300px. Even though my monitor is over 1900px wide, I rarely keep my browser window over 1250px because I find that lines of text become hard to read when the window is wider than that.
There are things that can be done with the CSS to make the site responsive to different page dimensions, but I don't know how customized the forum software allows you to be.
One thing you don't want to do is have a design that makes the site difficult to read for people with older browsers or smaller monitors (they're likely to be the least tech-savvy visitors and will just leave the forums rather than try to find a fix).
Like I said, those decisions should be made by photon and Bingo. How wide is too wide for the logo? Whatever they decide, everyone should submit their design within those parameters.
For example, you could do something like this: Post your final submission with your design over three background colours (red, white, black), contained within a rectangle 250x150px, and all submissions would look similar to this...
