My models are based on cap and body thread size. This allows for the grip diameters to be 1.5mm smaller then the cap and body thread.
Grip size is 10.5mm and 9.5mm at its narrowest. This is the smallest pen made with a #6 Jowo nib.
Grip size is 11.5mm and 10.5mm at its narrowest. This is the most common model make with a #6 Jowo nib
Grip size is 12.5mm and 11.5mm at its narrowest. The large size fountain pen
Grip size is 13.5mm and 12.5mm at its narrowest. The king size pen
Add a footnote if this applies to your business