Perseus was originally drawn for castings but its quite easy to fabricate which allows it to be made to any size and is a horizontal not the vertical you are now looking at. Myhobbystore and Reeves sell the drawings, if you click the drawing on reeves site it comes up a reasonable size to give an idea of whats required, but like the Hasbrouuck engines is in imperial. The plans were also free in the mag not so long ago. Though if I were doubling up I would go for the more detailed Unicorn.
The Hasbrouck engines look quite nice for barstock engines, there are a few builds of them on the net, I'll dig one out later.
As I said any of the Elmers engines can be scaled up but they tend to look a bit chunky, his plans can again be found on the net on sites like this
You could also up the size of the popcorn or overcrank engines that was in the mag not so long ago they are both in metric.
If you did want a nice looking large horizontal then Anthony Mount's latest offering must be about 18" long, It only uses two castings the rest if from solid/fabricated though the castings could also be made fairly easily again that will be in metric but will have some smaller detail.
J