Screen size (NOT PIXEL COUNT!) steps in grid

Good morning

I’m new to this community and I would like to ask for help from people with more expertise.
Let’s say I want to use an ion-grid and that I want to resize an ion-col BASED ON SCREEN SIZE, NOT ON PIXEL COUNT (I’M TALKING ABOUT REAL WORLD MILLIMETERS), what’s the recommended most straight-forward way to accomplish this?

Thank you very much in advance!