There are three different places you can edit the width of your forum. They range in difficulty of understanding and editing. We will go over each of them below.
First however, let's discuss the two types of forum width. In the examples below we will assume there are 3 different users using 3 different resolutions.
A variable width is set in a percent. This means that everyone who views your forum will see it differently based on their browser window's width.
Example of a percent width: 80%
You can see that each person is seeing your forum at a width that is a percent of their own resolution. Each person has roughly the same width compared to their screen size. They are seeing your forum differently from each other though. You can see that where words wrap to the next line is different for each person because they have more width if they have a larger resolution. This could potentially affect the overall height of your forum as a result.
A fixed width is set in pixels and will be the same for everyone who views it regardless of their forum width.
Example of a pixel width: 850px
In this example you can see that each person views your forum identically. Words wrap at the same place for everyone. People see things the same with regards to your forum wrapper itself. However, that being said there is a potential downside. If someone has a resolution that is too small your forum could be larger than their viewing area. And if there resolution is too big they may have a lot of empty space.
Where to Edit Forum Width
There are three different places you can edit the width of your forum. You may do so wherever you are most comfortable doing so. You only need to change it in one of these places and (as long as the theme's CSS hasn't been changed to break this) it will update in all three places.
The first and easiest place to edit your forum's width is on the Colors & Theme Creator page in the Themes area. Simply scroll down to forum width under Style options.
The second place you can edit width is in the Visual Editor page of the Advanced Styles & CSS section in the Themes area. In the Forum section under width.
The third place you can edit width is by changing it directly in your forum's CSS. Go to Style Sheet page of the Advanced Styles & CSS section in the Themes area. Depending on the CSS, on roughly line 56 find the attribute @wrapper_width and change the value to the desired width.
The forum software is designed to support resolutions of 1024x768 and larger. However you can make your forum width whatever you desire. Newly created themes are given a variable width of 90% by default.