Image dimension limit for a user contributor.

I have a question.
I’m crazy for this problem.
The problem is: When a user “contributor” insert a post with an image with the editor the image size “width and height” can be more larger of the layout. I want limit this for contributor user. I would want that even if a user insert in a post an image, the size is more small of the width fo the layout.
I use blix theme.
Please help me, i have try more plugin, but i don’t have found nothing for this problem.