If we assume a terraformed Ganymede has a global ocean roughly 800km deep, that food is bountiful, and that it has been populated by a variety of life from Earth, how large could these creatures grow restricted primarily by gravity and their ability to consume enough nutrients?
On Earth, the largest animal to ever have existed is the Blue Whale, currently in existence. This may give you clues.
Blue whales can weigh up to 250 tonnes, and swallow 120 tonnes of water in a single mouthful.
But the important question is: How did they get so big? Why don't all organisms get to this size? Whales are unique - they are mammals, filter feeders, and actually relatively recent (evolved 35 million years ago, with the current large species only 3 million years old)
The predominant theory is that whales evolved in a time that required:
- that there is a concentrated abundance of small filter food far apart (an ice age helps here) such that large bodies have an advantage to storing as much energy as possible
- the animals must travel long distances to get to sources of filter food - supporting large bodies
- upon arrival there must be an incentive to consume as much food as possible (a time limit till all food is gone such as seasonal change)
These factors support evolutionary pressure to create large organisms, at least in whales. On Ganymede, perhaps similar pressures would allow large marine animals too, but the primary driver is that there must be reasons to be large.
Without these drivers, then animals would be smaller and depend mainly on other evolutionary pressures - I imagine the depth of the ocean and gravity would be minor factors in comparison.