How to change the perspective/

Beginner trying to create a tower defense game. How do I get the characters to stay the same size when they are closer to the camera? Like in Mario3d, or the side scroller like Bad Dudes, or anything from the eighties. Am I stuck in serious 3d with Unity? I'd like the characters to not be super huge when they move closer to the camera. But I would also like some depth, so players can choose which enemy they want to attack.