How much vertical space do a rider and mount occupy together?
The two together occupy the same space (in all dimensions) as the mount alone.
Source - Rules Compendium p253, also the Compendium
Also, note that the previous page states that the mount must be at least one size category larger than the rider.
Remember that the space you occupy in D&D also includes the all space you can reach, i.e. the land area of a normal creature is a five-foot square. Nobody really takes up a five foot square but this is the space that your character can easily and quickly affect. In terms of realistic height of a mount with rider the height would be the approximately the shoulder height of the mount (for average horses this is about five feet) plus about half the height of the rider (assuming that the legs are longer than the torso, but in a sitting position) give or take the height of any saddle used. Therefore an average height of a human sitting on a horse is about eight to nine feet.