Make a copy of your themes full width template eg. page.php
file and rename it to bbPress.php
and you should be good to go. 
if we don’t have full width page template
Your theme has a full width page option viz:
Full Width Post
you can create a page which is full width, and just put this shortcode onto it :
[bbp-forum-index]
Then your forum will display full width.
I have same problem, I use Divi from ellegant themes. I have followed ur instruction.
But It doest work well.
I tried to change page-template-blank.php and page.php and rename both of them to be bbPress.php
Can you help me? 

I want to change the width bbpress forum as you see in the image the forum located in the left I want to center the forum list you can help me thank you
did you figure out how to make that?
.bbpress ARTICLE
{
width: auto;
}
Any ideas of how to make that:

?