Jellyfish larva, is said to have the smallest brain, other than the dinosaurs
the biggest dinosaur was amphicoealis fragillimimus and the smallest dinosaur microraptor or frutaidens
A dinosaurs weight would depend on the type and size of it. some dinosaurs weigh less than others.
All known dinosaurs had tails. However, birds are considered a group of dinosaur. Birds have the smallest tails of any dinosaurs.
Current holder of the title: Musasaurus, the "mouse" dinosaur
the smallest dog weighs .15 ounces
not that heavy
Some of the smallest may have lived in the trees, but if the question means "Did dinosaurs live in the oceans?" the answer is no. There were sea-dwelling reptiles, but these weren't dinosaurs.