Ideally, a properly matched pair of horses should be exactly the same height, but that was not always possible. A one or two inch height difference is not uncommon.
So, which side is the taller horse put?
Typically, if driving on the road, you want the taller horse to be towards the edge of the road - that way the camber will help the horses make up the difference. In other words, if you drive on the left side of the road, you put the taller horse on the left, and if on the right on the right.
If in doubt, the taller horse is put to the right - which apparently makes the pair easier to drive.