Buck,

Area of a circle (Pi) X (radius squared)

Area of a cylinder = area of circle X height

Pi is 3.14(rounded down)
Your radius is 15mm.
Your height(stroke) is 30mm.

3.14(pi) X (15(radius) X 15(radius)) = 706.5 square mm X 30mm(stroke) = 21195 cubic mm.

Since there are 10mm in 1cm, then this engine is 21.195cc(from your popsicle measurements.