How exactly is text sized?

I'm writing some scripts to make sure everything I draw in my GUI is scaled properly regardless of the resolution the game runs at. If a given icon takes up 10% on the screen at this resolution, it takes up 10% at this resolution. Basic stuff, really.

But I'm getting a bit thrown at making sure my text is drawn at the correct scale. And as I started going through a bunch of experiments to see what size my text needs to be, well, I realized this is probably information someone already has.

So, how does the game really interpret the numbers for font sizes?
When I change the font size in a GUIStyle, what does that value really translate to? What is consistent here?