I might be talking out of my ass here, defintiely not an expert, but I would imagine that the odds of a large dino skeleton surviving until today are much higher than that of a small one, so the perception that dinosaurs were typically big might be just result of survivorship bias. I always found it interesting that the largest known animal ever is the blue whale. A mammal, not a dinosaur.

Anyway, my money's on cephalopodae. I always thought their habitat would mean hyper-evolved octopus would make for a great astronaut. The big hurdle is that they don't raise their young or socialise at all, so they're sorely lacking in culture, but I heard there was a species of octopus in a particular area that has become significantly more social "thanks" to humans killing off their predators, thus making their lives significantly more leisurely.

Then again, dinosaurs recaliming their place would be pretty cool too, and I always liked jackdaws and other crows.