I doubt if you did anything wrong - the yellow veining and the slight crinkling of the leaves may mean that it has a virus of some sort - and there isn't anything you can do about that. A lot of dahlia stock out there is infected with one virus or another. It is possible that the signs are from a lack of nutrition. You might try a foliar feed and see if that makes a difference. If you have other dahlias nearby that are taller and greener - then its probably virus related.
As for height - there are quite a few dahlias that only get to 3 ft or even less, so it depends on the variety.
