RTX 2060 seems to be best at 1080p for all around smooth gameplay, diving into 1440p ultra on most games (not all games though), but you probably shouldn't put it in 4K because that would be too much for it to handle and you'd get stuttering gameplay.

I play on 1440

I run 3 monitors with my 2060 ultra XC, so not a 2060 super or anything like that. all 1080p, one is 144hz the other two are 60hz, and I only game on one of course, but I have very little issues at 1080p usually high to ultra settings, with RTX disabled, but if I enable RTX I usually have to bump other settings down to medium to low to compensate for the loss of fps. I imagine you probably could run 4k at mid/high settings but I wouldn't want to go over 1080p. 

My friend that has one plays on it in 1440 exclusively

1080p at medium/high settings will give you roughly 60fps in most games!

2060 does really well on 1080p. 200+ fps on esport titles with low settings