Link to tutorial on toggle between mm and inches and have object and dims remain same size wanted

I seem to have a mental block understanding Rhino when it comes to drawing an object in units inches, then needing to work with mm and keeping the object the same size (i.e imported or copy pasted to the item it will eventually be fitted to it is the right size on that item) and also stopping the dimensions going massive font size.

As others have said, they fail to see why a project needs scaling to hop from inches to mm then later back to inches. In real life if with imperial ruler you just reach out for the mm ruler, no scaling involved.
I also get miffed when the dimensions I have tweaked until happy with size go MASSIVE font height on me when doing so.

I know I can make a box by typing R7.5",8.7" when in units mm, and likewise R85mm,32mm when in inches, but how does one make a dimension of length 55mm when in project units inches.

I also wonder how to get dimensions to show mm if project in mm then they switch to inches when project is in inches.

I need to see a step by step of how to work between inches and mm, and no doubt a few others would benefit. Any tutorials or links welcome.