The BP250 Backpack Blower UT-08017-G can have either engine, 25 cc or 30 cc.

The differences in diameter and piston trajectory is around 2 mm; hence, it is difficult to measure.
You could try measuring the piston trajectory with a pencil inserted thru the plug hole; ~32 mm will correspond to the smaller engine, while the big one will read ~34 mm.

Guessing that the compression rate is around 1 to 9, the combustion chamber of the 25 cc (with the piston located at the top dead center) would take ~2.7 cc of oil (filling it up to the bottom of the plug hole).
The 30 cc would admit ~3.3 cc of oil in the combustion chamber.
(Flip the engine and empty the oil after the test).

Just a couple of suggestions on tests for rough estimates.