Steam supply depends on RPM and power output. The best advice is to size the boiler to the largest piston as if it were a single cyclinder engine and many compound engines run 100-150lbs. The pressure is greatly dependant on how much pressure your cylinders can safely handle, not enough pressure and your steam will condense before it gets into the last cylinder. With the D slide valve engine you cannot hurt it with liquid water, but there is nothing to gain from it. I'd start with 5 sqft of heating surface. Perhaps you live near a steam club? Steam clubs are full of friendly people who would be more than happy to steam your engine for you so you can get an idea of how much boiler you need.

Lastly I sent you a private message about this engine as I am building the same one, I'd be glad to let you know how much boiler mine took, once I am able to complete it.