The last time I measured I was about 3.5 inches.
Put your hair in a ponytail with as much hair as possible in it. Don't bother with the way it looks - the goal is to have most/all of your hair in there. If it means it sits smack dab on top of your head, put it there.
Measure the circumference of the ponytail. If you have bangs and/or you can't get all of your hair in there adjust according to how much of your hair you have measured.
i - thin (less than 2 inches/5 centimeters)
ii - normal (between 2-4 inches or 5-10 centimeters)
iii - thick (more than 4 inches/10 centimeters)
Edit: Oh never mind... I thought you were talking about your hair when you said "I have put 2 inches and under that means over 1 inch but not greater than 2." but you were just explaining the poll. Oops!