It all comes down to personal preference really. If I were to have a 1080p monitor aside a 1440p I'd go with something a bit smaller to maintain the same pixel density, otherwise stuff's going to be slightly sharper on one panel compared to the other. If you're switching between the two I could imagine it being noticeable. Especially if you had a 1080p at 27" aside a 1440p panel of the same size. By my calculations, though, to maintain the same pixel density on a 1080p panel compared to a 1440p panel, you'd have to go with something 0.75x the size. So, for 27" a 20"ish panel. That's a bit small tbf, scrap that. Could go with a 21.5" smaller 1080p and use it in portrait though. I could imagine that working. It'd be taller than the 1440p though.
I think 21.5 would just look so small due to the size of the 27" and the fact I use dual 24" at work..
I thought about going with a 4k monitor.. I wanted to keep price under $400 though so that pretty much is out. In the end I would be dumb and want one with gsync to see how that compares to my 1440p :P