Well it's aint straight forward is it? But here goes.
The overall length of the tube is 21.25" which is the length of the underside of the tube, from the stem tube to the back of the diagonal (with the AERO logo on).
The length of the top edge of the tube is 19.25", from the stem tube to the top of the diagonal.
From stem tube to front of seat post is 17".
There's a wee bit extra length as the front of the tube wraps round the stem tube.
Hope this helps.
Why do you ask?