Are you speaking of the width of the rails and stiles in general. Or are you asking how to change the widths of the materials that the rails and stiles are cut from?
You cannot set widths of board stock in the settings/preferences. The widths of the board stock is determined by the widths of the part that uses this material. For example if you have a face frame with 2 inch left and right stiles, a top rail of 3 inches and a bottom rail of 1 ¼ inches then the cut list will tally up the lengths of each width.
To change the width of the rails and stiles for the face frame. In Construction settings under the face frames tab click on go to page 2, here you can change the widths of the rails and stiles for the face frames on that cabinet. Do remember that if you have an existing mid rail or mid stile that you will have to take the cabinet into the face frame editor to change these widths.
To change the widths of the rails and stiles for the thermwood 5 piece doors in the door/drawer editor. Select a thermwood door, select build in house then click on the details for cut list button. This is where you set the coping depth and the panel inset as well as the panel board stock. Then click ok to accept. After that click on the layout tab here you can set your Rail and stile widths.
Hope this helps.