In the CSSMenuWriter interface, select the 'Box' tab and then in 'Dimensions, Margins & Padding' select the 'Item' tab. You can set the dimensions for the menu items (Main, Level 1, Level 2 etc) here.
Leaving a level's width value blank makes the menu fit to the width of the menu items' text. You can still add a padding value for 'all'.
Example here