Front Brakes Front brakes on a vespa are much like any other moped drum brake. The pads are measured by width and drum diameter. For more information on moped brakes in general, see this great page at Myrons Mopeds.
Rear Brakes The rear brakes use a specific pad that is mounted on the transmission inside the hub. The standard replacement is 135×16, as referenced on the page linked above. Some models came with a very skinny pad that can be upgraded to a wider one. Another upgrade option is a Kinetic rear pad, which is even wider.
The levers and transmission castings can vary, as well as other tolerances. If You are running an uncommon trans/wheel/pad combination, or just having trouble with full pad engagement, this rear brake lever mod might help.