Based on material from http://hdmipi.com
To get 1280 x 800 while using HDMIPi, changes need to the config.txt file to ‘force’ native 1280 x 800 mode. This will ensure full-screen utilisation in NOOBS and Raspbian, both in the command line and in LXDE.
sudo nano /boot/config.txt
Add the following to the end of your config.txt –
hdmi_group=2 # HDMIPi for 1280 x 800
hdmi_drive=2 # for alternative modes get sound
hdmi_mode=28 # 1280 x 800 @ 60 Hz Specifications
Comment out NOOBS auto-generated screen setup commands.
Note that for some displays these setting will stop ‘normal’ displays working at all