The width isn't a problem. The best length will be determined by your draw length (how far you pull the bands back), which bands you are using and how hard it is for you to pull. There's no point in having really strong bands if you shake like a geriatric fantasy falconer when you pull them, because you'll struggle to hit anything.
ive got double tbg its not hard to pull and aim with,stretches to about 15inch i don't shake with it and my ammo goes quite far i was only thinking they might need to be longer from seeing others cheers pal