How can I make camera automatically change its location ,so that it would adjust to the models of different size?

You can add this to your actor:

Image from Gyazo

You could even start the rotation only when the actor is clicked, rather than have it all go dizzy all the time.

Also I don’t know how to see the view
of the camera within the blueprint
when I play the simulation.

Possess the Pawn with the camera as suggested below, or Set View Target With Blend if you want to switch to any other camera.