I have that same motherboard. You cannot set AGP aperture size in the BIOS. Nor can you set any voltages, timings, or anything of that nature.
Aperture size isn't supposed to be a big deal, and most in-depth reviews I've seen suggest 64-128 megs anyway. So I wouldn't worry about it.
Just keep your system clean and drivers up to date. I believe you can get an updated version of the BIOS (3.28) from HP's site.