First attempt: load up X, then press Ctrl-Alt-keypad[plus/minus]--that should zoom in/out.
Second attempt: Edit the X server configuration file (/etc/X11/xorg.conf on my debian machine) as follows:
replace all occurences of (something like)
Modes "1280x1024" "1152x864" "1024x768" "800x600" "640x480"
(the config file on your system may be somewhat different)
Depth <THE SAME>
Modes "800x600" "640x480"
Then start up the X server, twiddle around with it, and whenever you feel like it, restore the original conf (make a copy of the file, or just comment out the changed lines, whatever you feel like).