The point sizes in AC are like text point sizes in any other software. It refers to the output size, taking scale into account. There is no need to calculate and adjust your text sizes for scale, AC does it for you.
If text will be displayed at different scales, you do need to be careful about the anchor point. When the text resizes for scale, that point will hold still and everything else will 'move'.
A point is about 1/72 of an inch, so 9pt gives you about 1/8". NB, different fonts may be different actual sizes at the same point size.
HTH,
