How can you figure out the cc of a dirt bike?

To calculate the cubic capacity of an engine cylinder you need to know the stroke height (from bottom dead centre to top dead centre) and the diameter of the cylinder bore - in centimetres. Using Pi (3.142), half of the diameter (ie the radius of the bore or "r"), and the stroke height ("h"), you can use this formula to calculate the cubic capacity of the cylinder. (Pi x "r"squared) x "h")
Your dirtbike doesnt tell you its more like what size it is. if its small its prolly a 150cc. if its medium size its a 250cc. if its large its a 450cc.