Changing Width of Post Containers…

I have a page that I am creating for a client, but I can’t seem how to make the container of the post and pages change past what it is.

Site I am working on is this: https://yellowjacket-media.com/wheelstream/toyotaolympia/stock-pr736/ You see that the form is cut off… but you see the page like this:

https://wheelstream.com/sunsetchevrolet/?p=3497

And the containters are full screen or the adapt to the width of your window to a certain point.

Basically, I need to figure out where in the theme code it is so I can make the containers change width.

Thanks so much!