I would love a height slider! But, the issue would be with animations (kissing, hugging, etc). Animations are created using a specific sized sim when they are animating it. If there were height sliders and your sim's head was only to the shoulders of another sim and they would try to embrace or kiss it would be totally off.
The sims uses pre-defined animations, they do not use procedural animations unfortunately. All sims in their age category are the same height, so like when sitting, there are butt and foot targets on each sitting surface for each lifestage ..... toddlers, kids, and adults.
