Correctly scaled risers that snap to grid

Hi. I’m trying to create an exact stage layout using different size risers to make the most use of an unusual space (in a church, pulpit is in the way and can’t block the aisles). So I’m using combinations of 2.4 × 1.2 risers and 2 × 1 risers. It would be nice if you could enter the size of your riser so it is to scale, then if you could make them snap to the grid, even better because they don’t currently (magnetic lines turned on).

Also, what measurement scale is the grid, imperial or metric? And how do I change it because it doesn’t measure up to any of the risers under ‘stage’.