Add the vga=XXX option to the kernel line, example below;
kernel /vmlinuz-2.6.22.3 ro root=LABEL=/ vga=790 quiet
Table of additional/other possible values;
Code:
Colors | 640x480 800x600 1024x768 1280x1024
-------+-------------------------------------
256 | 769 771 773 775
32k | 784 787 790 793
64k | 785 788 791 794
16M | 786 789 792 795