Regarding the barrow's height, I paid a quick visit to the page, Historical Wiltshire BARROWS - The Ancient Burial Sites of Wiltshire (_http://www.wiltshire-web.co.uk/history/barrows.htm) and the range in heights goes from four feet to the highest at twenty feet. Most seem to be between 6-10 feet.
I'm not sure if this is helpful, but I did a Google search and found a book by Rodney Castleden, "The Stonehenge People: an exploration of life in Neolithic Britain 4700-2000BC".
In it, Castleden claims a man from that time and region would have had an average height of five foot, seven inches tall.
A typical, thirty-year-old Orkney man was 171 centimetres (5 feet 7 inches) tall and a typical woman was about 12 centimetres (5 inches) shorter. A height of around 170 centimetres was typical of the long barrow men of southern England too
FWIW,
Gonzo