Can’t figure out what size my engine is

[Horsez]

I’ve been trying to figure out what size engine I have, but I don’t know how to find it and am not really familiar with this kind of thing. So, how would someone like me find out what size engine I have on my motorised bicycle?
Update: here are the pictures.

 

[Horsez]

...looks like a standard ole 66/80cc china doll motor.

66 (approx) US CC... 80 Chinese CC.

Thank you.