There's an easy way to figure this out for all types of speargun bands.
For a Euro style gun it's even easier since most of them use screw in bands although it's fairly easy to apply to loop bands and the tie your own variety as well.
Ex. 90cm Gun (90cm of band stretch) = Stock reccomended band size is 22cm for a 90cm gun. Why 22cm? 22 x 4 = 88 (pretty close to 90). + - 1-2cm within that won't make much difference depending on your preference.
Still, why 22 though? Any good band manufacter tries to find a happy medium between pulling strength required to load it and the power it gives in return. The widely accepted concensus is that three more times the length of the band is a proper stretch that doesn't cause good latex rubber to loose its elasticity too quickly (stretch out) while still taking advantage of the power it can put to the shaft as much as possible.
That's why 90cm guns come stock with 22's, 100 guns with 24's (24x4=96), 110 use 26-28's (28x4 112), 120 (30x4=120)....you get the idea.
With a conventional loop/tie your own system that goes through a round slot in the muzzle you should follow the same rule of thumb roughly. Take your 120cm gun and a 64cm piece of rubber, give or take a couple cm's for the section that runs through the muzzle and you still have two 30cm lengths that stretch out to 120cm at 4x their original length.
---------------------4 Lengths of Rubber on a barrel---------------------- is a good rule of thumb
If you can only pull your bands three times their length or maybe only two because it's so hard it's really crappy rubber with poor modulus. Dispose of it immediately or ask for a refund! If it's too easy to pull four lengths of it, it's also crap. Sorry if I offended anybodies rubber
Mark