If you experiment with Advanced movement, you can get a feel for how much a unit is.

One unit is not very long.
Normal walking speed is 600 units per second.

All Housing items move at a speed of 100 units per second by default (about the width of a magic counter/computer/timer which is 90 units cubed)
You can speed movement up to a maximum of 600 units per second with the speed up by 300% and speed up 200% spells.

(u for units)
Large Castle block floors are 400 u wide, 400 u the other way, and 25 u high.
Large Castle block walls are 400 u wide, 50 u thick, and 500 u tall

Player Detector radius is 100 u, and your wizard takes up a cylindrical radius of 75 u. (150 units from the front to the back collision boundaries of your wizard.
(This means the thinnest corridor/hallway you can move through is 151 units wide)