maxalge :
for 1440p you want a 27" monitor for the sweet spot on pixel density
for 1080p you want 24" or lower
Honestly that's a matter of preference and not a hard and fast rule. Also it depends how close you sit and how good your eyesight is. Looking at a screen yourself and deciding if you like it is probably always going to be the best option.
I have 1080 27" monitors and they are very nice. I have compared them to 1440P at the same size and yes it is better, but not as hugely so as some people would have you believe and once a game is playing it's not that easy to see the differences (except in GUI scaling for some games)
Also I don't sit close enough to see pixels and I don't find the text or icons too large the way some seem to. I wouldn't go any bigger than 27" on a desktop monitor without increasing the resolution personally but I also know people using a 32" 1080P screen and they seem perfectly happy with it. On top of which I game on a 60" plasma TV at 1080P occasionally from about 10 ft away and it's great.
So for me personally up to 27" on a desk 1080P is very good, from 27" upwards I'd prefer 1440, and 40" upwards I'd consider 4K - but not yet for me as I don't think graphics cards have quite caught up enough yet for 4K unless you can afford the very top end. Sittin gon a couch I'm perfectly happy with 1080P on a 60"