Theme: Terrafirma. How to disable sidebar and extend the page ?

Hi people,

I’m using the terrafirma theme.
https://wpthemepark.com/themes/terrafirma/

Now I want to disable the sidebar on 4 pages but not on the main page. Just like this one :
https://www.irakoplowitz.com

Now I do know how to disable the sidebar, but that will effect on all the pages including the main page.

But the most diffucult one is how do I extend the pages when I removed the sidebar ?

My site : https://www.detijdzevenbergen.nl