Here are the measurements you will need to give to Giovanni when you are at the armor shop. You will only need to enter this information in one time:

Gorget - 16
Cuirass - 1
Vambrace - 6
Tasset - 99B
Poleyn - 25
Sabaton - 6E
Helmet - 14
Pauldron - 5
Cuisse - Brown
Greave - 22 
Gautlet - Gothic