I'm creating a theme and currently use CSS to set width/height by percentage, so I want full sized images uploaded that can be scaled down. Problem is, when you create/edit a post and add an image, obviously it takes up a lot of space in the editor.

 

I'm wondering if there is some PHP I can add to change the way images display only in the editor?

I don't know about PHP code, but it is much simpler solution to just switch to Text editor instead of Visual editor.

In Text editor image is shown as one line of code.