Be sure to your graphics card drivers.
On laptops, try CTRL+ALT+F11.
Or, right-click the Desktop (Nvidia, Intel, AMD) and open your graphics card settings and look for the Scaling settings. Set it to Full Screen.
Sample Nvidia control panel, Set to Full-screen and click Apply
Sample Intel control panel, set to Scale Full Screen and click Apply
Sample AMD Catalyst control panel, set to Scale to full panel size and click Apply
Sample AMD Adrenalin control panel, set to Scale Full Screen and click Apply
