OKAY, found the solution:

Admin=> Appearance -> Customize -> General -> Custom CSS

Add this new code:

#wrapper-main {
float: center;
margin: 0 auto;
max-width: 1200px;
}

Then it works.

Also mobile phone work fine still.

How do you know it is about the #wrapper-main class?

You use Inspect to find it.

bbb

to change the width you will open css folder then style.css file and search for
( max-width )

WordPress

How to set max width for entire layout?

With the twentyseventeeen theme, how can I apply a max-width? Instead of the layout being full width, I would like to set it the max-width to 1000px and simply center the container if the screen width gets larger than that?

https://wordpress.org/support/topic/how-to-set-max-width-for-entire-layout/

#page {
max-width: 1000px;
margin: 0 auto;
}

NOTE!!!!
Wordpress theme does not have such a class name called page. So you need to find the real name by using Inspect element tool.