Why aren’t creatures and plants larger? Giant even?

The blue whale is the largest animal to exist ever, and we can see them today. And the largest organism alive is a forest in Utah where over 100 acres of trees are all connected at the root.

But to answer your question more specifically, the asteroid impact that wiped out the dinosaurs made it so that mostly small, warm-blooded creatures could survive. It was the age of mammals. We don't know for sure why, but the big theory is that small mammals lived in burrows or caves or other cool places because their warm-blooded nature would make them overheat in the sun.

So they hid when life on earth was going to shit, and emerged once things settled a bit. They were built to survive being small, and so their offspring and all the evolution that happened in the millions of years afterwards would have been built around a blueprint that made survival more likely at smaller sizes.

CO2 levels were much much higher back then. More plant growth. Warmer earth temps. More food. Bigger animals. Simple

I feel like things can only grow so big in accordance to their habitat. Back in dinosaur times there was far less of a population taking up resources

I believe we actually are fortunate to be alive at the time in the earth's history when the largest ever known animal is roaming the globe.

Weird shit happens

Look up a whale shark.