Insects

The size of currently living insects seems "just right" to us. However, if we moved back in time about 285 million years, we might get a shock.

An excerpt from the article 17 facts about insects
At that time, the Earth was inhabited by giant dragonfly-like insects, the largest of which was Meganeuropsis permiana. This insect had a wingspan of 71 cm (28 in) and a body length of 43 cm (17 in). A fossil specimen can be seen in the Museum of Comparative Zoology at Harvard University.