1280×960, 4:3. Hardly the numbers of an achingly beautiful, fully immersive virtual world being rendered on your screen, but these are in fact the preferred resolution and aspect ratio of pro CS:GO players.
And it’s not for framerate purposes. More frames can definitely give you an advantage, but it’s not like ‘Dupreeh’ can’t afford an RTX 3080 TI. The guy’s earned almost $2 million of prize money. Similarly, CS:GO is not a graphically demanding game at this point, what with being a 2013 release based on a 2000 mod.
This res and aspect ratio combo actually gives you a bigger target to aim at. When a player appears onscreen to ‘ZywOo’, ‘Twistzz’ et al, their model is being stretched over more pixels, since the game’s rendering a 4:3 image on a 16:9 monitor. Bigger target equals easier headshots. You just have to put up with some ugly.
Note that different monitors, graphics cards, and drivers have various approaches to rendering 4:3 in widescreen. You may have to do some tinkering to force the stretched image the pros use.