ARCHICAD so far doesn’t adhere to the standard of measuring font size in points.
Your only option is to enter text size in mm, whether that would be based on paper size (text stays the same size independent of the model scale) or model size (text size “relates” to your model, i.e. 100 mm in your model will always be 100 mm, no matter if you display your model at 1:100 or 1:1).
ARCHICAD measures the distance from the font base line to the highest point of the font (text on the left is from a placed PDF, text on the right is the matching ARCHICAD text):
Measuring this we’ve found the approximation of an 8pt font being equal to 2mm font in ARCHICAD.
Thus we can conclude that (roughly) for every point ARCHICAD counts 0.5 mm, i.e. 10 point font will translate to 2.5mm in ARCHICAD, 12 point to 3mm, etc.
Note however that this is a rough guide and there can be differences between various fonts. They should be minimal though. In the example below you’ll find Arial’s highest point doesn’t quite reach Helvetica’s.
Nevertheless we’d recommend setting your favourites to an easy to remember and check whole number rather than one with decimals which would exactly match a font’s size in points.