I wrote about the hydrothermal vent life on Enceladus in The Ice Moon Explorer, but that's not some crazy Clarkian act of prophecy: other people had suggested it before, and it would have been more bizarre if Enceladus didn't have hydrothermal vents.
Now let's have some fun.
The problem with ass cold places like ice moons, is that antifreeze oceans are still freezing cold. That's not an intrinsically energy rich environment, with steep energy gradients for life to use.
The polar seas are the closest labs we have for studying sea life in icy water. What we notice there is a trend towards slow-paced gigantism, like the Greenland shark. Polar gigantism was thought to be determined by high dissolved oxygen levels, but this is disputed.
If oxygen isn't the key, Enceladus is a great place for Earth-style, polar gigantism, at least throughout most of its biomes. If hydothermal vents can power a rich bottom ecology, that's plenty of food for deep sea drifters.
We don't have low gravity biomes we can study, but we have the next best thing - water biomes. Stuff in water, can get a lot bigger than stuff on land (I refer you to aircraft carriers). Water buoyancy effectively cuts the load structures have to bear. This means they can carry more load, and need less energy to move.
Enceladus has one percent (one!) of Earth's gravity, and it's a water world. How big can you get in a world-spanning ocean, with a percent of a G, and support from water? That's quite a fish tank. Crank up the hydrothermal activity (which not only provides heat but also nutrients), and there could be a not-so-cold water Amazon over there.
Bergmann's rule is basically that in hotter places, animals are going to be smaller. It's (partly) why Vikings are so big. It applies better to warm-blooded animals. With reptiles and insects, it works the other way. In hotter places, those get bigger.
What's special about heat and life? The clue is in the thermal regulation. In hotter environments, bugs and reptiles can heat up faster, and keep warm longer. This allows them to get more done, and be effective at larger sizes. Mammals get smaller, which increases their surface area to body ratio. This means relatively more cooling surface, and less mass to cool. A Viking will die of heatstroke before a pygmy will. And your cat, will outlast the pygmy.
Hydrothermal vents are more than likely where life on Earth first evolved, and the marker is how our cells harness energy. However, how far do the vents on Enceladus heat their local environment?
If there is ample heating, and cold blooded life, then vent fields will have large, fast moving creatures. Since the ground is where it's at, there would be a serious energy penalty for rising away from it, into cold water. So hot, giant, ground crawlers would prevail here. Worms are more likely (nematodes and even annelids are very simple multi-cellular creatures and so should be evolving everywhere they can). But giant crabs and scorpions also work. Maybe there are large, rich, vent fields near active fissures? Then a vent Amazon could have mega crabs!
If there is ample heating, and warm blooded life, we'd see the opposite. Smaller animals, who would be faster (small things can always move faster. Try to flap your arms like a humming bird, or play catch with a 9-year old. Neither works).
This may seem like a let down (and it is), but faster animals make dangerous predators. Ground hugging anemones that reach out and grab passing fish? How long are those grabbers? How fast?
Warmblooded animals would also manage in colder water. This may seem like a pointless investment given the heat is where the vents are, but crossing the cold would allow a creature to reach other vent fields. More fields to graze / hunt, means more food, and more environments to populate. It would be an expensive step, but Enceladus whales would do well once they got started, and become the dominant life.