Thanks to Charlie Mullendore of Antietam Classic Cycle for providing this information on the old Yahoo! Loopframe_Guzzi news group (which has now moved to Groups.io).
Bullet terminals are available in two common sizes: 4 mm (0.157 inch) and 4.5 mm (0.176 inch). The electrical connections on the loop frames use the 4 mm (0.157 inch) sized bullet terminals.
A great place to source these bullet terminals is Vintage Connections. They offer 4 mm bullet terminals with open barrels and strain relief. Very good stuff!