Hi Aleksandra,
You can change the width of single posts with some custom CSS.
To add custom CSS: Firstly set up a child theme or activate a custom CSS plugin. (If you have Jetpack installed then you can activate its custom CSS module.)
Am I right in assuming that you’d like to make the main content wider and the sidebar smaller? If so, the following custom CSS will do the job:
.single-post #primary {
width: calc(76.666% - 10px);
}
.single-post #secondary {
width: calc(23.333% - 20px);
}
In the above CSS: .single-post #primary controls the width of the post’s main content and .single-post #secondary controls the width of the sidebar.
You can increase/decrease the percentages to increase/decrease the widths. Remember, both percentages need to equal 100% for the sidebar to remain next to the main content. i.e. if you increase the percentage next to .single-post #primary then you’ll need to decrease the percentage next to .single-post #secondary by the same amount.
Let me know if that’s helpful or if you were looking to achieve something slightly different.
Thanks!