How to enable the map unit function in "Sphere_Miller_Cylindrical" CRS in QGIS?

Question

I am making world maps, for varying purposes, and one of the requirements is to label each country with her name. Normally I used the map map unit function in order to adjust the labels size to the map scale. This time I was asked to use the "Sphere_Miller_Cylindrical" CRS, not the regular "WGS_84" CRS, so I change the project properties.

However, once I made this changed the label were gone.

What I need to do in order of making the label reappear?

Answer 1

The map unit size is set respectively to the CRS units of the layer. I head changed the project CRS, but not the layer CRS. So, I need to match map unit CRS to the project CRS. I opened the expression string builder and wrote this function, so the unit map size will be calculated in the "Sphere_Miller_Cylindrical" units.

This solved the problem and the label reappeared.