I don’t think height would matter so much as much as electronic detection, maybe put sensors on the top or underneath with cameras positioned every so often where any border guard could take a look if one of those sensors are triggered. If there’s going to be no sensors then to me a wall it going to be pretty much useless. They’ll get through anything they put up, look at the tunnels the drug gangs dig.
That’s why IMO everything for 12 miles south of the border should be a no man’s land. The U.S. military should have 100% permission to blast anything and everything that moves in that zone.