Kamama
The Omer web page has a good reference on the band sizing,
Spearfishing and Freediving Equipment - Latex Bands - Power 20/16 - Technosport Inc.
A good rule of tumb is 1/3 to 1/4, so if your gun is 100 (euro size), since american guns are mesured on the entire lengh of the stock and euro guns are mesured on the size of the barrel, so if you between 33 to 25 cm depending on how much effort you want to put also the band thickness can be a factor as well. I bought some RA bands, I had to make them a little longer since they are so tuff.
