I have a 144hz monitor but play in windowed mode, do I have to play full screen to use 144hz?
No that’s not necessary. Your refresh rate set in your Windows, nVidia or AMD settings (depending on your card) will be leading on windowed mode. So if you have those on 144Hz, you are good. Just turn it to 60Hz or whatever low value in either one of those places for testing/comparison and you’ll notice that ingame it will have the same refresh rate and thus mushy screen.
Regardless of refresh rate, playing in windowed mode has a slightly higher frame to screen latency but that’s negligible.