Hi mylinux,
To increase the size of sidebar, you could have 2 common ways.
First, decrease the size of article, then increase the sidebar.
Second, increase the size of whole site, so that the size of sidebar increase according because the width of them (the sidebar and article) set to % not px.
So i will do with second method
Try adding this to child theme style.css file
.site {
max-width: 1140px;
}
with the code above, the width of whole site grow and of course, the sidebar too. But if you want the sidebar increase more. You can continue add this
.site-content {
width: 60%;
}
.widget-area {
width: 31.8457%;
}
I assume that you want these value, but play around with these value until you get your desire result
NOTE: You can use these default value to get better idea
.site {
max-width: 1040px;
}
.site-content {
width: 64.1042%;
}
.widget-area {
width: 27.7417%;
}
Good luck!