In the Folder Options dialog, where you can see the column widths, click on the Display tab and you'll see an Auto-size columns option. It's on by default (like in the screenshot below), but you want to turn it off if you want to fix the size of your columns.
That's probably all you need to change.
(Remember the Folder Formats: Quick Guide if you need steps on saving the change so it applies to all folders.)
