There are also photos of him at various ages in airports and auditoriums, such as standing under a ceiling that the height has been measured and he's standing under it.
In France, I think he started out being billed as 7 feet. He got a little more popular and he was 7'1. A while later, he was 7'2 then 7'3. He came to Canada and was 7'4. He was even 7'5 for a while but it didn't catch on.
Most "wrestling scholars" based on meeting him when he was young or having photos from that era with landmarks to compare him to estimate he was between 6'10" and 6'11", probably closer to 6'11". On his death certificate, he was listed as 6'9" but that was likely due to the spinal curvature he suffered in later life.