I have both small and big dogs, is there an area where dogs of different sizes can play together?

Yeah they have multiple areas for different sizes and energy levels

That depends on both your dogs personality and temperaments. I have taken my small dog in the large dog section, and it was ok... but I watched closely for any signs of aggression from the large dogs. Try your dogs first in the large dog section...I think that would probably work out better for you and your "fur kids".

Usually the little dog goes in with the big dogs.

Not technically. If your big dog isn't too hyper, I've seen people with big and small dogs take both dogs into the small dog area. But that makes some of the people with small dogs unhappy because they're afraid of the big dog hurting the little ones.