Show Camera Preview

You can change the viewport size of the camera. It won't show in the scene view though, only on the game view.

You could select a camera in the inspector, then lock that inspector and use another one for other objects.

You could probably also try and change the script provided in the following answer:

http://answers.unity3d.com/questions/312472/how-to-change-size-of-camera-preview-in-editor.html

select the cemara in the $$anonymous$$erarchy，then just expand the "Camera" component in the inspector