I don't have knowledge of this particular reconstruction, but it depends. Stone walls can often be reconstructed directly by taking a designated section of rubble and rebuilding the wall to see how high it is. Some walls in the original site may also have retained their original height depending on construction, environment, materials, etc. Everything else is about constraining the range of possibilities based on evidence. It takes a certain base width to support x height in the local soil, the wall had some human purpose and probably wasn't overly tall for that (height = $$$), etc.

All of this is to say that reconstructions are largely a series of educated guesses in most cases. That's generally fine as the goal isn't to be perfectly accurate, but instead to give tourists an engaging historical experience.