We picked up a new snail today. The new one is a red one... same size as our blue one.

They are both in a 5 gal tank at the moment before being moved back into the 65 gal.
The blue one has been chasing the red one and climbing all over her, off again, then back to climb on top of her shell again.

Is this agressive snail behavior or mating behavior or something else?

Welcome to the world of Slime! Truthfully it's a pretty normal behavior in snails to crawl all over each other-especially if it's a boy. Those things will try and mate with everyone.

The two snails are about the same size. Would the one being climbed on have a way to let the other know enough is enough. She has closed her flap once that I saw and does not really seem to get away. They also tumbled down from the top of the tank a time or two.
I’m thinking about putting them in the larger tank so if she desires, there is more room to separate. Every few minutes one the blue snail is on top of the red.

Could be mating behavior. It's nothing to worry about, snails are not agressive. If you see the red one trying to fling the blue one off it's shell, then it is mating for sure.