I had the same problem. Very hard sometimes with the nVidia cards to get the resolution right by futzing directly with xorg so I like to use arandr for this kind of thing. (This is one of the few times that I find the GUI stuff better then the command line approach.)
pkg_info tells us about arandr:
# pkg_info -d arandr
Information for inst:arandr-0.1.7.1
RandR is designed to provide a simple visual front end for XRandR.
Relative monitor positions are shown graphically and can be changed in a
Maintainer: Edd Barrett <email@example.com>
You can do all the same stuff with arandr that you do with xrandr but I find it a bit more useful to use arandr when dealing with a couple of monitors but that's another story.
So install and use arandr to setup your display the way you like.
When you fire up arandr set your resolution under 'Outputs->VGA->Resolution. You should see your different choices xorg is offering for that card and probably VESA will show up rather than VGA. Then use the 'save as' button to export these settings as a script. Usually I dump that script somewhere like ~/.screenlayout/fullscreen.sh then invoke that script somewhere in my .xinitrc with:
and then when I fire up X the resolution I want is in place. If I'm feeling really adventurous I go in and modify xorg.conf afterwards but I've been lazy lately and don't muck with that much.