The right size EarJellies for you are the largest EarJellies that you can roll out into a long, narrow cylinder-on-a-base shape and slide successfully into your ear canal. You may be able to use the next size down, but the seal may not be as effective.
If a pair of EarJellies are too large for you, you won't be able to slide them into your ear canals and they won't seal properly. Instead, they will bunch up every time you try to insert them and their bulbs will look short and fat when you remove them from your ears.
If a pair of EarJellies is too small for you, you will be able to insert them easily into your ear canals, but they won't seal properly. You will continue to hear sounds at nearly the same loudness as without the EarJellies and they will not keep out water. When you remove them, they will have almost their original bulb-on-base shapes rather than being molded to the shapes of your ear canals.The easiest way to determine which size EarJellies you should be using is to purchase the Variety Package. It contains one pair each of Small, Medium, and Large. Try Medium first, but be ready to switch to Small or Large if you have problems with that size.