The band itself is just over 1.25″ (3.2cm) wide.
The base is 5.25″ (13.3cm) for the “Small” and 6.0″ (15.2cm) for the “Standard” size, with a 1/2″ (1.3cm) strap for affixing the watch.
For both sizes, the leather from the buckle to the watch is about 3.25″ (8.3cm) and from the other side of the watch to the end is 5.0″ (12.7cm).
Because the band sizes are identical for both the “Standard” and “Small” size, choose your size by the base, which is 3/4″ (1.9cm) shorter in the small. In other words, the 6″ (15.2cm) or “Standard” size will cover more of your arm, the closer your arm is to 6″.
So generally, choose “Standard for an arm width above 7” (17.8cm) and “Small” for anything smaller than 7″ (17.8cm).