Look at the pics I posted. I created red "bars" that I used as rulers. If you create a bar that is 4.56 meter long and that is centered at the same coordinates than your capsule, then you can use it to check the scale.
You could also make a "disc" (short cylinder), maybe it's more convenient to check the scale of objects with a circular base.
Make sure you delete your "tools" before exporting, else we'll see them in Orbiter.