You could try the following to check wether or not 800x600 is the maximum screenresolution;

1.) Open and run your project
2.) press F11 to get the debug panel so you can see which is the current resolution (top left in white text)
3.) press F5 to change resolution to a higher setting

See if this works. If it does, then there's a code problem I think. If it does not work, does your monitor (or laptop screen?) support screen resolutions higher than 800x600? If not, it probably won't work, not 100% sure what would happen though.

Cheers