The individual pieces in the arch (the wedge shaped) are called Voussoir, to determine the size of each piece, you measure the inside of the arch, then the outside and divide into equal amounts this gives you your size of each Voussoir.
Another great post Deano, just to add to my earlier post, the correct term for inside of arch is Intrados the outside is Extrados. This I remember from my days at Manchester college of building (circa 1968) A bit of information that has come in handy a few times. Never thought it would at the time. Just shows there is no such thing as useless information.