How can I change the font size for all written posts at the same time?

Edit the styles.css to give the text the size you need.

If you don’t know how to do this, you need to provide a link to your site for specific details of what to change as there are thousands of themes and the css will be different for each.

I apologize, link!

OK you’re using the default twentyten theme, so you need to either rename the theme or (preferably) create a child theme to prevent any changes getting overwritten when updating WordPress.

Then to change the body text size on the posts you can modify the value of the entry-content class:

.entry-content {font-size: 14px}

Thank you, sir! What do you mean by create a child theme? (I’m sorry if that is a dumb question, I’ve never done this before)

Child themes are the best way to make modifications to twentyten without editing the original twentyten files. The codex article at https://codex.www.ads-software.com/Child_Themes explains it well.

For the most basic implementation of a child theme to make the modification I described above, make a new directory called live-your-best in your wp-content/themes directory, then in that make a file called style.css with the following content

/*
Theme Name: Live Your Best
Description: Child theme for the Twenty Ten theme
Author: Your name here
Template: twentyten
*/

@import url("../twentyten/style.css");

.entry-content {
    font-size: 14px;
}

Then go into your appearance panel and select the new theme.