Add to the Modes lines in /etc/X11/xorg.conf, unless you using the XVesa flavour? You may also have luck adding "SCREEN=1024x768x24" to your GRUB boot-loader 'kernel' line.
It normally runs at the best resolution it can find, though.
yeah, Slitaz ignored the boot option for 800x600x16 for me too and defaulted to a very high resolution. i hand-configured what is below into /etc/X11/xorg.conf as a subsection to Section "Screen":