I would say a majority of people would disagree with what I'm about to say, but at 1080p, I find 27" to be a sweet spot. I will admit that 1080p on smaller looks sharper, but for me, at a comfortable seating distance, I find 27" at 1080p to be more ideal.
If you can get a look at monitors either that a friend owns, or on display in a store, of different sizes, I would highly recommend doing so. I admit my eyesight isn't as sharp as many other people, but seeing varying sizes at 1080p will let you know what feels right to you.