i assume you have GDM as your Display Manager - check the /etc/gdm/gdm.conf for which dpi resolution it starts - i think it's 120 dpi - you can try to set it to a lower number - i have 96 dpi and it works fine.
when you use the XDM, you should check the /etc/X11/xdm/... files
you will find a line with /usr/bin/X11/X -dpi ###
try to set different numbers here - Windows - as far as i know uses 75 dpi, Mac uses 96 dpi, my Debian Installation had 120 as Default.
when you change the dpi size, you maybe have to change fontsizes and so on in your desktop enviroment, under gnome2 i didn't have to change anything - i don't know how about kde3
hope i could help
there was a related question, to find here:
it's more detailed and so on