The standard way of describing the height of a horse is by measuring how many 'hands' high it is. At first this way of measuring may not have been absolutely accurate. Somewhere back in history however it was agreed that a hand should be standardized as 4 inches. Anywhere above 16 hands is considered a pretty tall horse. A pony is under 14.2 hands high. On the other 'hand' the horse is 14.2 hands high and over.
A dry stone wall that is below 4 feet (12 hands) in most cases is not that impressive. It would be more like a pony than a horse. A tall wall looks elegant. It looks like it has a purpose other than to be sat on. A short wall looks like people were afraid to build any higher, or that it wasn't finished yet.
The taller a wall is built the more of a statement it makes. We have to take every opportunity to build proper tall free standing walls when ever we can. It may mean really taking the extra time and energy to educate people about the merits of building walls that have this traditional height based on them being a type of barrier for livestock containment. If it can't hold grazing animals in, it isn't being honest to its roots.
Even a retaining wall needs to have some height to it to look good and more importantly for the physics of it to work in terms of it having mass and being able to act like a dam to hold back the soil. A short dry stone retaining wall along a driveway definitely doesn't have as much impact or as much 'curb appeal' as a tall one.
In any case life is just too short to go round building short retaining walls.