Both "adults" in the tweenies are played by a 6'4" man, the others are normal sizes. If you notice you never see max and Judy together except in a split screen effect. When they go outside you will notice the the child tweenies are puppets. This is because the adult is played by a normal sized actor due to scaling problems with members of the public that they meet.
The names of the costume characters are on the end roller and the voices are not by the people in the costumes. In fact the voice of Jake and Doodles is by Justin Fletcher from Something Special and Tikkabilla.
Hope this helps