There’s a coffeetable book, uh, around here somewhere (or maybe I left it somewhere else I frequent) with some very nice photos of loads of various little-studied (gee, I wonder why) pre-Islamic antiquities in Saudi Arabia; but I just doubt these size estimates.
Yes, it isn’t the easiest part of the world to travel round :) - I managed to get up to the Nabatean burial site at Mada’in Saleh in Saudi a few years back, which is stunning: almost in the same league as Petra. Talking of books, the best (by far) that I’ve come across pertaining to the history of this part of the world is ‘Arabian Sands’ by the British explorer Wilfred Thesiger. Some fascinating mentions of ancient tombs he came across in the Rub’ Al-Khali (Empty Quarter) in Oman, which are quite likely still sitting there unexplored and uninvestigated.