Use the CSS code below: 

div#wpdcom {
   width: 100% !important;
   max-width: 100%;
}

Put the code in the Top Admin Bar > Customize > Additional CSS > "Additional CSS" textarea, save it, delete all caches and check again. 

Please don't forget to press Ctrl+F5 (twice) on the frontend before checking.

My website HerbSpeak.com has been having an issue with full-size comment boxes since the Wpdiscuz 7 update came out.

I am using WpDiscuz version 7.0.7. The only other plugin I have that affects the comments area is WP User Avatar, version 2.2.8

I am using a child theme of Divi Extra on WordPress 5.5.3

Under Settings > Styles and Colors > Custom CSS code, I have the following code inserted which has doubled the size. This is from the first time I reached out regarding this issue when the 7 update first came out. Doubling the size was good enough for me at the time, but I'm in a space where I'd like to fully resolve the issue now.

I have played with the margins in both percent and px and it renders the code inert (but my coding experience is very limited.)

.comments-area {
width: 100% !important;
}