Export an Excel spreadsheet to fixed-width text file?
I think the closest you can get from native Excel functionality is Save As | Formatted Text (Space Delimited) (*.prn). It will automatically determine the widths and insert spaces to pad to that width as necessary.
Beyond that you need to have a macro or other add-in that will let you do more.
If you have Office Professional, you can open your Excel file in Access, and then Export from Access. Access will let you specify a fixed-width layout for your exported file, and gives you extremely granular controls for specifying those widths.