Seems like many still use those reverse distance x resolution chartz that are spread around. Where it is bad to have a 4k TV if the monitor is to small, or that it's bad to have a big TV if you sit to close.
DF mentions many times on the Switch that 720p looks sharp on a small screen in portable mode. This is no different the bigger the screen the more noticable the pixels get.
I gamed on a 42inch 1080p screen and it looked super sharp. Going to 55inch 4k screen, i can tell if i am running 1080p instead of 4k.
Honestly anything around 30inches i am happen to drop resolution for preformance however on a big 4k TV i wont, due to how noticable it is.
It doesnt take a rocket science to see a difference, however it depends if its worth the preformance gap.