Hello, in wp-admin you can go to Appearance –> Editor and click custom.css on the bottom right. There you can set custom css styles.
For posts font size and also for the pages, place the following line in there, set font size as you like and update file.
.entry-content p {font-size: 18px;}
If you want it for the posts summary on the home page to be the same size, it should be like this:
.entry-summary p, .entry-content p {font-size: 18px;}
