I had to do it this evening : here is how to proceed :
Let’s start with an existing list in Sharepoint, a task list for instance :
Start Sharepoint Designer and open up the corresponding view (AllItems.asxp, here)
Make sure the view shows up in design mode :
Right Click anywhere in the view and select the menu option “Convert to XSLT Data View”:
The DataFormWebPart will display the view;
You can now visually modify the list width :
Save the page; refresh your page in the browser.