How to change/control table/box size?

My problem;

I have a WP-Blog, with a contact form on top and all the posts underneath it. Right now the text in the actual blog posts exceeds the table/box (whatever it’s called), meaning there’s text outside the box, to the right. I want to change the width of the box/table so it fits all the text. I guess the text thing is even a result of me somehow changing the width of the text area but not the actual table/box.

So the main question is; how do I make the box/table wider?