I want to change the text size of the blog titles in the side bar. When I change “blog post title” size, it also changes on the blog page, blog grid etc. So this is not a solution, because I only want it to be change in the side bar.
You can change the font by means of Advanced typography option, which allows to choose the item from a drop-down or insert your custom CSS class. Please navigate to the Theme Settings > Typography > Advanced Typography.