The next thing you need to do is adjust the command line options for
the X server. You'll want to use the -dpi switch which
specifies the display resolution in dots per inch. As a lot of systems
use high resolution displays these days, chances are they'll be working
at 100 dpi.
If you start X from the console command prompt, type
startx -dpi 100.
If you use xdm for graphical logins, you'll want to edit your
/usr/X11/lib/X11/xdm/Xservers file, which will have the
command line for the Xserver in it. Mine has the line
:0 local /usr/X11R6/bin/X -dpi 100 -gamma 1.6
More information is in the X and Xserver man pages.