Sorry to be so late, just saw the thread and answered anyway.
The combustion chamber volume does not have anything to do with the capacity, but it does dictate the compression ratio.

Swept Volume (capacity) = D x D x 0.785 x Stroke. (dimensions in Cm)
Where D = bore, S = stroke.

CR (compression ratio) = swept volume + combustion chamber volume, divided by combustion chamber volume.

(to measure the stroke, measure from crankshaft centre to crankpin centre and multiply by 2).

The 28cc model would probably suit a 1/4 scale model maybe 84" wingspan.

You probably know all this by now anyway!
Cheers, Will.