At the risk of spoiling my own grant application for that research trip to Geneva, I've found this simpler way:
According to a website I found about the cars in Tintin books, that blue taxi that Tintin catches on page 7 of The Black Island
is a Ford Zephyr Mark III.
In the first frame of the second line of page 7 (ie: frame B1), Tintin is standing next to the taxi, The top of his head is a litte lower than the height of the taxi's roof, but he's stooping just little to talk to the driver. I reckon that if Tintin stood up completely straight, the top of his head would be pretty much level with the height of the taxi. (If you look at that picture, you'll see what I mean.)
According to this
Ford Zephyr enthusiasts club website, the height of a 1962 Ford Zephyr Mk III is 1.454 metres, or 57.2 inches, which is 4 feet 9 (and a bit) inches.
This is considerably shorter than most people's estimation of Tintin's height on this thread. Is Tintin really 4 feet 9 inches, or did Hergé and his team draw that Ford Zephyr too big? Or is Tintin stooping more than I thought? Or has the website I used got the make of car wrong?
Personally, I'm not sure that 4 feet 9 inches is unfeasibly
short for Tintin, given that Hergé saw him as being a boy between the ages of 14 and 18. Maybe all the estimates earlier in this thread really are putting Tintin (and the other characters) too tall.
I guess we need to find a picture of Tintin (or any of the main characters) standing next to another known vehicle, to double check the figures derived from my Ford Zephyr gauge.