this is done by using frame buffering (in the kernel).
If you kernel has frame buffer support, you can edit your LILO or GRUB conf file to use frame buffereing and set the mode you want. the setup is different for different files. in LILO you waill add
vga=xxx where xxx is a number from a chart
in GRUB you'll add the same but you would have to add it to the line that says
kernel (hd0,0) .... vga=xxx
now the problem with this is that sometimes it doesnt work and when u boot you'll get a completely black screen. so to test this out, you'll want to create a new line in your file (lilo or grub.conf) so that if it doesnt work, you have a good configuration to boot into. i think the alterntavie for grub is to add it as an option when you boot up (i think you hit 'a' or some key before you choose an OS)
here is the chart:
640x480 800x600 1024x768 1280x1024
8 bpp 769 771 773 775
16 bpp 785 788 791 794
32 bpp 786 789 792 795
cant get it to line up but u should get the point. dont be too agressive with the values. try 771 or 773 or even 769 1st. And like i said, you need kernel support for this but it might already be built in so it's worth a shot to try it out. and please make sure you add an second entry in your grub file that matches your current default boot system with just the difference of having the vga=xxx part so that if it messes up u can boot the system back up with the safe configuration.