The right way to do this would be to find the config file that is changed, and edit it to the correct resolution. But I'm not an XFCE user so you'll have to look for this yourself :)
Another, desktop-agnostic way is to change the resolution using
xrandr. Assuming you can still boot up your computer and use Ctrl+Alt+F1 to access a command prompt, you can type this to change to automatic resolution:
xrandr --display :0 --output VGA1 --auto
After the command has finished without any error you can use Ctrl+Alt+F7 to go back to the GUI and it should have the right resolution. Don't forget to change the resolution again, so that the configuration will be saved.
You may need to change
VGA1 to the name of your display output (type
xrandr --display:0 to see the outputs).
--auto isn't smart enough you can change it to e.g.
xrandr --display :0 --output VGA1 --mode 1024x768
, which will change the resolution to 1024x768 (again, type
xrandr --display:0 to see the possible resolution modes).