[NTG-context] what defines the font size?

Dear list, sorry for this basic question, but what defines the size in a font? I mean, we can take TeX Gyre Bonum and TeX Gyre Adventor at twelve points. x-height is different, M-width is slightly different. I thought it was the same A-height, but it isn’t. If two fonts have the same size, I think there may be a dimension which has the same length in both. Which one is this? Sorry for the basic question and many thanks for your help, Pablo -- http://www.ousia.tk