If you're using Grub to boot, add "vga=773" to the end of the kernel line in /etc/grub.conf. For example:
kernel /boot/vmlinuz-2.4.9-13 ro root=/dev/hdb1 hdc=ide-scsi vga=773
If you have Lilo, add the line "vga=773" to /etc/lilo.conf under the image section. Then run "lilo -v" as root. The next time you boot the resolution will be changed.
There are quite a few more resolutions to choose from, so take a look at "/usr/src/linux/Documentation/svga.txt" for more choices if 773 doesn't fit your needs.