I have just picked up on this thread.
1. The GWR issued a weight diagram, A.32. This shows bogie wheel dia 3'6", driving wheel dia 6'0" and tender wheel dia 3'10".
2. Wheel bases - bogie 5'6", driving wheels 8'3", rear bogie wheel to front driving wheel 6'6 and a half". Tender 2 X 6'0".
3. I know of no official drawing but one might exist.
4. There is a very strong likelyhood that the late Mike Morton-Lloyd made a drawing in 7mm scale.
A Happy New Year to all