The scan will tell you head circumference, abdominal circumference and femur length. (The 12 week scan also tells you crown to rump length but won't be an accurate predictor of birth height or weight at that stage).
For my growth scabs they then do a calculation with these 3 figures which guesses the baby's weight.
So if you and your partners height is mainly down to the fact that you both have very long legs, then it may be a good indicator. However... If, like me, you are tall but with a very long body and proprtionately short legs it may not be because the scan will not take torso length into consideration.
I was a 10lb baby so I've been pretty paranoid about this... But my growth scan a few weeks ago predicted a baby of about 7lb. However, like I said, all my height is in my torso which the scan won't pick up on so I'm expecting a heavier baby than predicted.
I needed growth scans because my bump has measured below the 10th centile with the tape measure for most of my pregnancy. The midwife said this is really common in tall ladies, particularly those with long bodies. It sounded like she was basing this on anecdotal evidence rather than research, but it kind of made sense to me as the baby has more room to 'hide'. Im 39 weeks now and can still wear all my non-maternity tops and dresses! So if you're the same you may well find you end up having a growth scan further down the line which will give you a better indicator.
But yeah, I agree with the pp that head circumference is much more important and I've been watching this measurement like a hawk!!!
There is also an old wives tale that the width of your pelvis is the same as the length of your foot. So big feet often = wide pelvis. I have absolutely no idea if this is based on any science whatsoever but as a girl with size 9 feet im clinging onto all hope!!