So far, no example of this toy has come to light either, but at least we do know its origin.
Guessing the size again depends on the wheels shown in the illustration, which in this case seems to clearly indicate the Tender has wheels like the Cricket, Forklift etc. Comparing it to a Cricket would make the Crash Tender at least half again as long, say 125 mm - 5 in or so. Which would still justabout allow it to fit on a blister card to be sold with the other smaller vehicles.
Blister card in orange
Not available as GA
A pair of vehicles briefly appearing in a Thunderbirds comic strip by Frank Bellamy, published in TV21 nr 108 of 11 February 1967.