Question About Units

Ok so I'm incredibly confused as to how units work in Unity. I've got a 2d game with a scene that's 1920x1080px. I want to place a square that's 120x120px in the upper right corner of the camera space, so I would assume that I would take half of 1920 (960) and half of 120 (60) and subtract 60 from 960 to get 900. I'd think to do the same with the y coordinates and get (900, 480), but since everything is measured in units, my simple math doesn't mean anything. I've already read around and seen that there's this thing called "Pixels per Unit" and I've tried messing around with it, but it doesn't really help with my problem. I guess what I'm basically asking is how do you determine how many units wide is the orthographic camera if the screen size is 1920x1080? If I can figure that out then I should be able to do the rest.