Change width of posts?

I know how to change the width of the forum itself by going to admin/colors, but i need to know how to change the width of the actual posts. they are so wide, it's hard to read without scrolling to the right.
How do i change the width of the posts?

thanks.

Can you show an example screenshot?
It might be caused by an image in the post.

No, it's not because of an image. It's just regular text.
I've already changed the width of my signature, but that doesn't seem to change the width of the post at all. Some posts are wider than others. (that may be due to paste-ins). The question here is: where/how do we change the width of the posts?
thanks for any help.
Jae