How to get CS:GO 4:3 with black bars or stretched?

The display aspect ratio and use of black bars is up to individual preference. There isn't one optimal way for all. Some pro players prefer stretching, black bars, or full screen. 

If you are using Nvidia graphics card and prefer to adjust by display, open the control panel > to perform scaling on "Display" > select "full-screen" mode

If you are using AMD graphics card and prefer to adjust by display, open the control panel > go to “GPU Scaling > turn off “Enable” button

To learn more about XL Monitors