re: Small/Big Mammal/Reptile
The data points are pretty scattered. There were many small (chicken-sized) dinosaurs, as were their descendants, the birds. And after the big (and presumably warm-blooded) reptiles were wiped out, some mammals got huge (giant sloths, 10 foot beavers, house-sized ruminants, etc). Also, mammals did not "replace" the dinosaurs, there were mammals contemporary with dinosaurs almost from the beginning. What I find fascinating is how the dinosaurs who were so successful for so long at filling so many niches were exterminated while many much more delicate animals (like amphibians and modern reptiles) survived to be relatively successful into the modern day.