Why were dinosaurs so big?

Lots of oxygen on the planet. 

The dinosaurs didn't think they were big.  To them, they were just the right size.

Not all of them were. There are only hypotheses why some did. Overabundance of food and hollow bones allowing them to grow to great lengths and heights. Another is since they were reptile-LIKE, they may have been able to grow indefinitely during their lifetimes.

An over-abundance of CO2. 

By no means all were very large, but size may have helped the largest, which were herbivores.

I may think of it have a lot of oxygen and carbon dioxide in the air.
-Dinosaurs that are so large are called terrible lizard.