I suggest you get rid of it by giving it back to the pet store you bought it from. Chinese algae eater when grown up, will suck the slime coat off your betta fish and show very aggressive behavior. I just returned my Chinese algae eater yesterday.
As for the size, they can grow up to 12 inches long and the width of a mans underarm. They'll only grow to this size when provided a huge area. Usually, in aquariums they grow to about 10cm in length. When they are that size, pellets wont be enough, and your betta's slime coat will look very delicious to them.