In the menu.1st file in /boot add the following to your kernel lines.
vga=771
I am assuming you want it at this before you start the GUI environment.
Other codes you can use are:
Code:
Color depth | 640x480 800x600 1024x768 1280x1024
-----------------+-------------------------------------
256 (8bit)| 769 771 773 775
32000 (15bit)| 784 787 790 793
65000 (16bit)| 785 788 791 794
16.7 Mill.(24bit)| 786 789 792 795
If it is in X, then set it as your default once its started, it should remember.