Reddit reviews Invertebrates - Second Edition [Hardcover]

TL;DR: Microscopic animals can be very small, down to 50 μm length. The main issue is the diffusion of oxygen.

Smaller animals reside solely on the transportation of oxygen -the main resource in animal metabolism- by diffusion, this is because the surface:volume ratio: smaller organisms (smaller volume) have more surface than volume, this helps the diffusion of oxygen. Diffusion is very effective when the diffusion path is less than 1.0 mm.

Here are some groups of very small microscopic animals:

Group | Length (μm)
-----|---------
Rotifers | 50, 100-2000
Gastrotrichs | 75, 50-3000
Kinorhynchs | 150, 100-3000
Hydra (genus of cnidarians) | 500-20000


As you can see, the most basal groups of animals are pretty small. For comparison, bacteria can be 0.5-5 μm in length.

• Source: Brusca & Brusca, 2003. Invertebrates, 2nd ed. Sinauer Associates, EU.