The sizes of bags range significantly. Smaller bags are available in quantities as little as 2L in capacity, while larger bags are available in sizes as big as 120L (this is essentially duffel bag territory). The length of the activity, if you're traveling: will it fit in the allowed carry-on size range?, and the amount of gear you're hauling about will all obviously affect the size requirements you have. https://outdoorgearreviews.org/best-waterproof-bag-for-swimming/