There are some customization options for the Blog page that you can find in X > Theme Options > Blog.
However, those options are limited to the layout of the blog page depending on the stack you have chosen.
As for the things you are aiming to do, you can achieve them through custom CSS.
I tried checking the site that is attached to the license in your licenses page but it seems to be not using X.
Please check the items below for some suggestions for you to get started. Please note that is based to an Integrity stack blog page. There is a chance that the codes will not work if your setup is different:
.single-post .x-main {
width: 800px;
}
.single-post .entry-wrap .entry-content p {
font-size: 16px;
line-height: 150%;
}
I am not sure you mean by line width as there is no CSS option for this.
You might want to check the Responsive Text feature of X to have the font size more responsive on mobile.
Please note that the CSS suggestions above overrides the theme setting and if you require further customization, it would be best to get in touch with a developer as this goes beyond the scope of our support.