Designing an engine...
So this is the way I went with my latest engines (P2, Atlantic, Gladstone) to get the dimensions about right.
Tried to get a bit further away with the camera and zoom in so the outer parts of the picture aren't distorted by the angle (compared to the paper)
The grid is from brickguns
from this page
I used photoshop to enlarge the grid and lay it over the engine's drawing. Really helpful to get the proportions right!