Modern Vespa

Am I correct in that a certain size piston is sized to fit a certain size bore ?
IE: the bore for a 52.4 piston is 52.4 and allows for proper clearances and there is no real need to measure the piston at certain points ?

In theory the person who did the boring will have added the correct amount of clearance which can be measured with feeler guages.
I recently put a 53mm piston in a bored air-cooled Honda and the clearance was 0.04mm or 1.5 thou.
The piston felt tight when dry fitted but it seems to run OK.

When buying a new P and C set, the clearance should be correct, but it never hurts to measure. When a cylinder is bored out, there are usually published specs for what the clearance should be. For machined parts, the best fit is always obtained from measurements taken from the parts you in hand have. Some builders are anal to the point of providing the machinist with the micrometer they used to measure, though I have not heard of this for Vespas.