Hey There,
It kind of depends on what you mean by "fixed width". Do you mean making all of the columns each have their own fixed width, or do you mean making the cms as a whole have a fixed width? If the latter, the easiest solution would be to add the following line to your additional.css template:
Code:
.vbcms_content {width: [pixel width here]px; margin: 5px auto;}
If you want each of the columns to have a fixed width, it gets more difficult. The last time I did that was about a year ago; I don't remember the steps exactly, but it involved editing the html of the cms grids by hand. Of course, I could have been doing it wrong

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-Nick, SEOvB Developer