I sit about a 1500mm from mine.
Id' definitely go to 32". I used to games on at 1200p on a 28" and loved it and I much prefer it over a smaller screen of the same res, 1440p is only going to be an upgrade over what I already used to and liked. I wouldn't go 4k due to the poor refresh and slower performance. Refresh and frame rate is very importance to me and I value it over image quality. I like screen sized of 28" and above personally, at least for gaming. Office type work smaller screens are fine to me.