In article ,
) wrote:

Think of it this way - a model airplane engine can be made to run with
1/20 of a cubic inch (.049 cu inch to even .010 cu inch), but piston
engine aircraft became impractical above a few thousand HP. That is
the range of practicality for a piston concept.

It is certainly possible to build much larger piston engines than that.
How about http://www.bath.ac.uk/~ccsshb/12cyl/