How do you resize the column width of a list in Trello?
In Trello's current implementation, this is not configurable: lists are programmatically sized to be between 300–210 pixels wide depending on the available space.
(Substitute the 500 with your own desired pixel width.)
You can execute this at your browser's console, or save it as the following bookmarklet, to click whenever you want some more width:
Edit: Another option for the bookmarklet to request you the actual desired size is:
It will not validate the answer, so if you enter something that is not a number it won't work.
This is not an available option and so the widths of columns in Trello are fixed.