Concerning pistons, you'll have to find your cylinder size before you go ordering. There will be a stamp (A, B or C) on the back lower edge of the cylinder. This is the size. You will then need a piston matched to this.
However, this is not always true, and really applies to new or nearly new cylinder as if your cylinder is worn then there's a good chance than on 'oversize' piston will be in order.
Speak to Andy and see if he's got anything handy.