Do you want to control minimum width or maximum width? As Ivo said, the "Main menu width" controls maximum width, so you can set it as you want to a width beyond which if the program name goes, it will be truncated and shown as ....
To set the Minimum width, there is not a setting available but you can use a very easy trick to set it. Go to Customize Start Menu tab in Settings, locate any item in the left column, for instance, "ProgramsMenu". Double click "ProgramsMenu" and insert as many spaces as you want in its LABEL after the word "&Programs" to increase the width as required, and finally end the LABEL again with an ampersand (&). e.g. if your ProgramsMenu label is: &Programs, change it to:
&Programs (multiple spaces) &
(varying the space as needed, just making sure it ends with an &). That will make the minimum width fixed.
Let me know if this helps.