Skeleton — inside, outside, or water

Design axis

Four ways to hold a body up, and the choice sets a hard ceiling on how large the creature can get.

The four, and what each costs

Endoskeleton — bone or cartilage inside. It grows with the animal, so there is no size limit from the skeleton itself. Vertebrates. Sharks use cartilage rather than bone: lighter, more flexible, and it does not fossilise well, which is why shark teeth are most of the record.
Exoskeleton — chitin worn outside. Excellent armour and leverage, but it cannot grow. The animal must moult, and is soft and defenceless each time. Combined with air-tube breathing, this is what keeps insects small.
Hydrostatic — muscle working against enclosed fluid, with no hard parts. Worms, jellyfish, and the octopus arm. It allows deformation nothing rigid can manage — an octopus passes through any gap its beak fits through, because the beak is the only hard part it owns.
Lignified cell walls — the plant solution. Stiffened cellulose, built cell by cell, holding a redwood upright for a hundred metres with no skeleton in the animal sense at all.