How do you knwo your frame size to obtain an accurate bmi calculation?

Height and Weight: If you have those two measurements you can calculate your bmi - many sources available on the internet. The number alone is not an interpretation in itself. It needs to be taken into consideration with other factors.

Doesn't matter: A Body mass index is just a reflection of how much mass is present in a body of a stated height. It doesn't care what frame you have, the number is the same. The listed normal for the index does assume a certain values of amount of bone/muscle at different ages, so age is a factor.