Ian
Westbury designed a boiler suitable for his diagonal engine. It is drawing M10 - Trident Major Internal Flue Boiler.
This is a large engine and should be driving paddles much larger than those shown. Should be capable of driving a six foot model. If you manage to get it into the Director as drawn it will probably sink!
Steam is POWERFUL as I discovered with my first steamer. Lots of folk have built Stuart Double 10's with the intention of fitting them in three foot long craft, but it really doesn't work.
Unless you are intending putting it in a large model, I guess the best bet is to make a nice job of it and put it on a shelf where you can admire it, then build something much smaller.
Malcolm