Hi,
I got website set to 1200px width however I would like to have a bit more narrow blogposts. Is there a way to change just blog posts sizes?
Yes that is definitely achievable with the Layout Element module.
https://docs.generatepress.com/article/layout-element-overview/
First, you must create a Layout element.
After creating the, go to its Content tab and set its “Content Width” to your preferred width and set the content area to “Contained”:
https://docs.generatepress.com/article/layout-element-overview/#content-1
You can then go to the display rule and set the location to where you want the content width to apply.
In your case, you can set the Display Rule Location to “Posts – All posts” for all single posts.
https://docs.generatepress.com/article/layout-element-overview/#display-rules
You can even apply it on posts of specific categories if you want.