Tribsauk: I measured a 53 Speed Twin sprung hub frame which I believe is the same as a rigid frame.

Top tube from front casting to wheel casting is 16 9/16 inches. (ignores slight bend in tube)

Lower tube from brake pedal casting to wheel casting is 9 5/16 inches.

As well the drawing in the link given above does not show the bends very accurately.

Gordo