How does the sizing work on this bike?

Question

http://www.harryhallcycles.co.uk/directory.asp

it says available options

16 18 19.5 21

what measurement is this?

Answer 1

That is the measurement from the top of the seat tube to the spindle of the bottom bracket.

Do NOT confuse this with the size of the wheels.

Answer 2

This is the number of inches long the bottom bracket is or something like that.  I am 5'7" and my bike is a 19" and it is a little to big.  I recommend a 16 or 18 for you.

Answer 3

its the size of the wheels... that is the only thing that they have sizes for on bikes