My Mountfleet 'Highlander is 33 in. x 9.5 in. at 1:24 Scale. At 40 in. this would equate very closely to 1:20 . Highlander would have been one of the earlier Puffers built to fit the 20 M Locks, although I have read that they were actually built to 66ft!! (Sorry Bob!)
My Funnel is 140 mm x 35 mm, which for you Paul would be something like 170 x 43.
John