Dimension question - how to obtain a size?

I have a friend that wants to do 3D printing of objects to go inside of other objects, but they would have to be of a given size. He wants to build house interiors and such things for small models, and print them.

But, I’m not sure how to create a model that is a foot long, or even tell how long a model really is.

Any thoughts on that topic would be helpful.