They are both the same species, Bala Shark
. Both fish will grow to 14 inches, some authors say close to 16 inches. Assuming they are well cared for of course, in small tanks they will probably be stunted, prone to health issues, and die early.
Stores use small, medium, large to describe the particular fish in the tank, since prices usually vary from least expensive (small) to more expensive (large) because someone has had to feed and raise the larger fish and when you buy it you pay for that.
We have fish profiles here, second tab from the left in the blue bar across the top, or in posts you can click on the name if it is shaded, like Bala Shark
is, and see that fish's profile. Have a read; this fish needs a lot of room, at the very least a 6-foot tank. Being a social shoaling fish, it needs to be kept in a group, not singly or 2, 3. Otherwise the social structure will be lacking and stress to the fish results, meaning more health problems.
Don't be afraid of asking questions. We were all newbies once.