When I reached my full adult height I was 5'9". When I got measured for the first time in many years in my mid-30s, I was 5'8"! I had them measure again, making sure I was standing as tall as I could. Yup, 5'8". Same result when I got measured again at the age of 41. Soon thereafter I started taking yoga, just once a week, and also incorporating some of those moves in my post-workout stretches. When I was measured again a year later, I was 5'9" again. *whew*
Yoga will DEFINITELY improve your posture and carriage, but it can also lengthen your spine. No dramatic differences, mind you, just keeps you from getting more compacted than you should be. It lets you reach your true full height, in a way.
Another thing you can do to help with posture and carriage is make sure you are doing lots of good upper back / shoulder / chest exercises in weight training. This will help reduce slumping and give you the appearance of a smaller waist, to boot.