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Let's use the sea turtle and the terrapin as an example, need I say more? I used terrapin because like the mermaid which is half fish and half girl so a terrapin would serve as an excellent example.

Generally speaking, species usually grow much bigger in seawater conditions. However rules are meant to be broken but never for the mermaids, does it not?

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  • $\begingroup$ @FabioBeltramini: the density is different and so is the oxygen level so osmosis depends on the blood composition etc. $\endgroup$ – user6760 Dec 1 '15 at 9:20
  • $\begingroup$ Seagoing mermaids may evolve to become large enough to deal with oceanic predators like seawater crocodiles, sharks and orcas. $\endgroup$ – Thucydides Dec 2 '15 at 2:35
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Evolution supports increasing size if it confers some sort of advantage to the large members of the species. Sauropod dinosaurs became larger and longer to effectively crop leaves from taller and taller trees, for example.

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. Larger Mermaids would also have proportionally larger flukes, allowing them to swim farther and faster to track and capture prey. And of course, since the oceanic environment already supports some pretty large and dangerous predators, like seagoing crocodiles, great white sharks, giant squid and pods of Orcas, being larger provides more protection, or at least a greater ability to fight against a predatory beast in the oceans.

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There is nothing to prevent differing mermaid groups being differing sizes, while still being "human-size".

We have groups of humans of varying sizes, e.g. African Pygmy tribes and Nordic countries with average heights approaching 2m tall.

"Human-size" is an enormously wide range.

It would be easily possible to have porpoise-sized lake mermaids and dolphin-sized sea-mermaids.

Typically, the size of a water creature will relate to the sizeof the body of water it is living in. Compare the size of the Great Lakes of North America to a dribbling brook.

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Really this choice is down to you. It's certainly true that many sea-water creatures are larger than their fresh-water counterparts but that does not have to hold true for mermaids.

If you want to have differently sized mermaids then that's fine, just keep the populations separate and let drift and selection do its thing. Generally ocean mermaids would grow larger over time and fresh water ones smaller.

On the other hand if you want them to all be similar sizes then just have regular mixing and inter-breeding between the two groups and the selection pressures will be reduced enough to keep the two groups fairly homogeneous.

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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.

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