From the Reeves site
Twin Cylinder Engine
By Edgar T Westbury
With Stephensons Valve Gear
Directly Coupled to 5 3/4" Paddle Wheels
Described in Model Engineer Vol 113-114
Length: 10 1/2"Height: 6 1/2"Width: 14" (minimum) across the paddles
Cylinder Bore: 3/4"
Cylinder Stroke: 1 1/2"
plus the boiler tank and other bits and bobs

this will require quite a reasonable hull
would love to see this if it takes off