Go to forum admin > Options > Content settings – ‘Post Content Width’. Follow the instructions in the help popup (I see there is an error in there that refers to the ‘styles’ panel. It means the input field in that section.
Give that a try…

Thanks for the fast reply!

Unfortunately, it didn’t work. After following instructions, the panel correctly changes the width to the size of my forum.

But it’s okay, hmm. i don’t think the possibility of someone inserting a very long topic without space is very low. 

Also very impressed that you guys already thought about this and put it into the option. If you come across a better solution, please let us know

I don’t understand. It did NOT work but it DID change the width correctly. That should work. Unless you are using a browser that does not support CSS3 word-wrap…