The only way that I know to alter the command line screen size is by setting the vga= option from lilo (if you're using lilo as a boot manager, that is).
If using lilo, you can put vga=ask into the lilo.conf file, recompile and reboot, and you'll be asked at boot what resolution to use. For more information on this, consult man lilo.conf.
If you don't use lilo, you can use rdev/vidmode. I assume that the changes will require a reboot to take effect (don't ask me why it's this way, I think it's stupid, personally).
To alter the video mode, first run rdev to list the device on which you need to make the change (should return /dev/hda1, or something similar).
Then, run either:
rdev -v /dev/hda1 3 (3 to set to ask mode)
vidmode /dev/hda1 3
The -v makes rdev act like vidmode. You can learn more about these by consulting man rdev.
Hope this helps!