If you are still running in 16-bit Real Mode, then you can use the BIOS to switch video modes to a specific graphics mode (mode 13h), or you can define your own. (i.e. 1024x768x32)
However, as long as you are in 16-bit Real Mode, you are going to be extremely limited in what you can do once you are in graphics mode. So, you may want to consider switching to a specific graphics mode, and then switching to 32-bit protected mode.
You can find more information on the wiki pages:
How do I set a graphics mode