Resolution is more important than size, so I'd go with 2 monitors, assuming all monitors are same res. But if picking between say, 2 HD or 1 4k then go with the 1 4k. 1 4k monitor is like having 4 HD monitors in a 2x2 pattern.
Personally I have 2 28" 4k monitors one on top of each other. Do most of the work on the main one and then have stuff on top that is secondary. Like when I'm coding I'll usually have my code and file system view on the main monitor and then consoles and whatever I'm working on, on top. So I can write code, then go up, hit refresh. Or if I need to run a script I'll do it from top in the console etc.
For size, I find 28" is a sweet spot for 4k. At work we have 3x 27" 4k and middle one is HD 27" and I find stuff can be a TAD small. That extra inch makes a difference. (that's what she said)