You need to edit you /etc/X11/xorg.cong file, find the specs on your 15" lcd Vert. and Horz. also the max res. and enter them in your xorg.conf.
boot to recovery mode use what ever editor you like and make the changes, you need to be root or sudo. Here's a copy of the part you need to edit.
Identifier "Generic Monitor"
Modes "1024x768" "800x600" "640x480"
You only need to edit the parts I put in bold print, in the modes section put what ever resolution you want to be default to the left.
You never specified what Linux flavor you are using, if you are using Debian you can also start in recovery mode log in as root then give the command dpkg-reconfigure xserver-xorg
and just follow the instructions.