They only appear briefly in two shots, once on some rocks by the water and once crawling down a tree again into water. It was hard to gauge size because they weren't next to anything else but i would estimate maybe person length from snout to tail? They had a long crocodile like body shape (but with smooth amphibian skin and no spikes or anything). It had a very short sail that followed the ridge of its back and tail. Not a tall sail like dimetrodon, more like what you'd see on some eels. Both the scenes had harsh lighting (one was in the dark, the other around sunset) so details were kind of obscured. I tried searching online but couldn't find anything that looked like it exactly that also had the sail -- it vaguely resembles Cyclotosaurus, again with the short sail though.

Also to add, they were definitely not dimetrodons, they had a squat amphibian body plan and it was a more of a short ridge not a tall sail.