You need to know the current screen resolution (for full screen mode) and separately you need to know the window resolution (window mode) Are there any standard functions in C # or in XNA to get this information?
ps for the window mode you can make it possible to select the window resolution, and these values can be manually entered as options, but for full-screen mode you need to know the possible correct resolutions: my screen has a resolution of 1366x768 and someone has 1920x1080