Are your mermaids civilized?
While you had a lot of good answers, I'd like to point out one thing that counteract natural selection: civilization. Just look at humans. Sure, there is a lot of diversity among us, with Asians being on average shorter, Africans on average taller. There is difference in build, skin color, bone structure (roughly), etc... Those difference came from different environments but overall, if you were from another species, there isn't as much as a difference between two humans than between a sea-turtle and a terrapin. (For starter, they doesn't have the same limbs)
So you could realistically keep human-sized sea mermaids and river-dwelling undines if they could adapt to the challenges of their environment. A few examples regarding Thucyclides answer:
In the oceanic waters, Mermaids, like other sea creatures, would probably evolve into larger sizes to change the surface to volume ratio, conserving heat and minimizing energy expenditures
Wear clothes. Sea-clothes. Special mermaid clothes that increases buoyancy.
Larger Mermaids would also have proportionally larger flukes, allowing them to swim farther and faster to track and capture prey.
Pack tactics. Use net, outsmart your prey. Communication and planning should help a lot in hunting big game.
And of course, since the oceanic environment already supports some
pretty large and dangerous predators, [...], being larger provides
more protection, or at least a greater ability to fight against a
predatory beast in the oceans.
Band together and build shelters. Use weapons to compensate your lack of natural weapons. Teach your potential predators to avoid you. Heck, domesticate some sea lions or something, they would make great sea dogs in a story.
All the examples above are how humans coped with their status in the food chain to raise on top and protect themselves. Now, natural selection hardly happens to us, and the interbreeding tends to flatten the differences.
TLDR: You had a lot of good answers to justify putting different sized mermaids, here is a reason to justify your choice if you want to do the opposite: civilization.