Hi Bazyle,
The two main shafts are 1/2" diameter (I think? they are Silver Steel).
The Main Block is 3/4" thick x 2.5" wide x 3.5" high.
Also…if you look back, at Ian's picture, the shaft with the Triangular shaped piece attached (bottom left of picture) swaps out with the plain second (or middle) shaft in Ian's picture of the main assembly…to give you a sliding Tailstock support for a Pinion on an arbor. 
Whereas, on the photos shown in the link posted by John (Journeyman)…aka my one…It shows the attachment fitted with a short (potentially sacrificial) support/holder for the cutting of a Pinion Head.
All in all…it is a very simple…(the best ideas usually are, IMHO)….but very effective attachment (designed by Colin Walton) that doesn't take up a lot of room…and being so compact…can be used on a wide range of machines.
Personally, I am certainly really pleased that I managed to find one
))
Regards,
Adrian
Edited By Adrian Nicolson on 04/04/2020 14:09:18