By default the CDE font aliases that ship in the distribution do not point to fonts that are available on Linux. If you have access to the gsfonts-x11 pack on Linux you can tweak the aliases to use Helvetica and Courier instead of fixed.
Edit the file /etc/X11/fonts/misc/xfonts-base.alias and replace the "-dt-interface-*" font entries with the lines below.
After changing these aliases and restarting the X server, you should now be able to pick your font size choice in the dtstyle control panel.
"-dt-interface system-medium-r-normal-xxs sans-10-100-72-72-p-61-iso8859-1" "-adobe-helvetica-medium-r-normal--10-*-75-75-p-56-iso8859-1"
"-dt-interface system-medium-r-normal-xs sans-11-110-72-72-p-64-iso8859-1" "-adobe-helvetica-medium-r-normal--11-80-100-100-p-56-iso8859-1"
"-dt-interface system-medium-r-normal-s sans-12-120-72-72-p-73-iso8859-1" "-adobe-helvetica-medium-r-normal--12-120-75-75-p-67-iso8859-1"
"-dt-interface system-medium-r-normal-m sans-13-130-72-72-p-87-iso8859-1" "-adobe-helvetica-medium-r-normal--14-140-75-75-p-77-iso8859-1"
"-dt-interface system-medium-r-normal-l sans-15-150-72-72-p-100-iso8859-1" "-adobe-helvetica-medium-r-normal--14-140-75-75-p-77-iso8859-1"
"-dt-interface system-medium-r-normal-xl sans-18-180-72-72-p-114-iso8859-1" "-adobe-helvetica-medium-r-normal--18-180-75-75-p-98-iso8859-1"
"-dt-interface system-medium-r-normal-xxl sans-21-210-72-72-p-123-iso8859-1" "-adobe-helvetica-medium-r-normal--20-140-100-100-p-100-iso8859-1"
"-dt-interface user-medium-r-normal-xxs serif-10-100-72-72-m-60-iso8859-1" "-adobe-courier-medium-r-normal--10-100-75-75-m-60-iso8859-1"
"-dt-interface user-medium-r-normal-xs serif-11-110-72-72-m-60-iso8859-1" "-adobe-courier-medium-r-normal--11-80-100-100-m-60-iso8859-1"
"-dt-interface user-medium-r-normal-s serif-12-120-72-72-m-80-iso8859-1" "-adobe-courier-medium-r-normal--12-120-75-75-m-70-iso8859-1"
"-dt-interface user-medium-r-normal-m serif-13-130-72-72-m-90-iso8859-1" "-adobe-courier-medium-r-normal--14-140-75-75-m-90-iso8859-1"
"-dt-interface user-medium-r-normal-l serif-15-150-72-72-m-110-iso8859-1" "-adobe-courier-medium-r-normal--14-140-75-75-m-90-iso8859-1"
"-dt-interface user-medium-r-normal-xl serif-18-180-72-72-m-120-iso8859-1" "-adobe-courier-medium-r-normal--18-180-75-75-m-110-iso8859-1"
"-dt-interface user-medium-r-normal-xxl serif-21-210-72-72-m-140-iso8859-1" "-adobe-courier-medium-r-normal-*-20-140-100-100-m-110-iso8859-1"
"-dt-interface user-bold-r-normal-xxs serif-10-100-72-72-m-60-iso8859-1" "-adobe-courier-bold-r-normal--10-100-75-75-m-60-iso8859-1"
"-dt-interface user-bold-r-normal-xs serif-11-110-72-72-m-60-iso8859-1" "-adobe-courier-bold-r-normal--11-80-100-100-m-60-iso8859-1"
"-dt-interface user-bold-r-normal-s serif-12-120-72-72-m-80-iso8859-1" "-adobe-courier-bold-r-normal--12-120-75-75-m-70-iso8859-1"
"-dt-interface user-bold-r-normal-m serif-13-130-72-72-m-90-iso8859-1" "-adobe-courier-bold-r-normal--14-140-75-75-m-90-iso8859-1"
"-dt-interface user-bold-r-normal-l serif-15-150-72-72-m-110-iso8859-1" "-adobe-courier-bold-r-normal--14-140-75-75-m-90-iso8859-1"
"-dt-interface user-bold-r-normal-xl serif-18-180-72-72-m-120-iso8859-1" "-adobe-courier-bold-r-normal--18-180-75-75-m-110-iso8859-1"
"-dt-interface user-bold-r-normal-xxl serif-21-210-72-72-m-140-iso8859-1" "-adobe-courier-bold-r-normal--20-140-100-100-m-110-iso8859-1"`