For Text+, how does the Size property work?

For Text+, what exactly is the Size property a measure of? Different fonts with the same Size property have different actual sizes when rendered. Such things are usually relative to one of the dimensions of the frame, but that's not the case here.

I need to make text that is consistently a specific percentage of the frame height, for any font, but the Size property has me stumped.

Can anyone tell me how this works? Thank you!