Millimetres would be the easiest and most accurate measuring scale.
Here is my largest VFT so far this season, and one of my largest ever:
one and 7/16 inches.
just a shade under 1.5"
1.5 to 2 inches is considered the realm of the large VFT..
I still have never seen one documented over 2"..
(reports of 3" VFTs are just wishful thinking.)