I installed DietPi on my RaspberryPi 3 and my display hast bars top/bottom because it is 1280x800 (ex laptop display with touch)
Please make the resolution custom set-able just by choosing X-X Custom and then just ask the user for pixel numbers 
BTW How can i set it permanently to 1280x800 if i have xfce as desktop manager, would be nice it is persistant over boots
The problem is that indeed RPi sets resolution based on a set of config.txt entries. Please see here hdmi_group and hdmi_mode section: https://www.raspberrypi.org/documentation/configuration/config-txt/video.md
Setting the framebuffer dimensions only, affects console size but not graphical screen resolution.
If you select the desired resolution via dietpi-config this should be valid for the desktop as well? But I am not sure about possible interferences between kernel/boot and desktop resolution settings from GUI.
yes you set if in config.txt once then it should work
nano /DietPi/config.txt
And then Edit the Resolution works nicely after reboot