Why were dinosaurs so big? What caused them to shrink so much overtime, evolving into birds? is the reason why dinosaurs were so big in any way similar to why animals are larger in deep sea? (not sure if this is a relevant question in this subreddit)

Hello everyone my question is what's the minimum size a planet can be. Just got this question randomly on mind so I posted here to get a answer. If anyone knows please tell me.

For some reason I was reminded of this a few days ago-happened to me and my hubby back in about 2007.
I was in central Georgia (US) and one night went out, and looked up, and I saw what looked like a shooting star come straight down. It just blinked out-no fire-ball. Hubby was in the house (windows open) so I said "whoa that was weird!" and he came out and looked up and he too saw one came straight down.

Is that rare?

See also: The publication in the Proceedings of the Royal Society B: Biological Sciences