Stirlingengineforum.com

The most noted units were 12 and 16 inch diameters with strokes of 24 and ~40 inches.

I assume the short strokes today are in the interest of RPMs, but there's a lot to be said for running more slowly.

Pretty much everything is inter-dependant: materials; temperature difference; power; speed

Those numbers don't look very different to what I would expect for modern power-producing machines - LTD models are at the opposite end of the spectrum due to that pesky "Low" in the name.