Years ago I was in a small-engine repair class and another student asked about a large, single-piston engine. The instructor said there would be too much lag time between cycles for the engine to be practical (or to possibly even stay running). He said you would basically get spurts of power during the combustion cycle. Sort of a BANG-pause-BANG-pause-BANG.
There are large, one cylinder engines in use out there. In glider school in Germany they used an old, average-sized agricultural tractor with a one cylinder diesel engine to set up their winch tow system and for other jobs around the airfield. You could easily hear the individual "pops" of ignition at normal operating RPM's and a slow chuga-chuga-chuga at idle.