Units & Rulers

This sounds like something you would do with crop marks in InDesign.

Just to clarify the basic concepts: a raster image file doesn't have a size, only pixels. That's the unit Photoshop always works with, at the core it has no concept of what a millimeter is.

The print size is determined by whatever pixels per inch-value you have assigned. Any metric measurement in Photoshop is a recalculation from the ppi number.

Make a feature request under the "ideas" subsection to get this recalculation applied in the tool of your choice. If the request gets upvotes, it will be considered, the more, the better chance.