I'm not sure I even know of any units to measure with other than the tiles here. Unfortunately, some of the trophies don't have a plaque, and some like Chokethorn need more wall space than the plaque takes up. Most seem to be the two-tile size, but a number are significantly larger. Some have no plaque at all.

I have the Engine Guardian, and it caused a lot of problems trying to place it in the entrance corridor of Mathiisen Manor. It reached halfway across to the other wall, and couldn't be sunk into the wall or it popped out into the adjacent room. It ended up pushed into an external wall not too far away.