I'm using WP edit plugin, how can I add custom font size?

Im currently using WP edit to choose font style and font size for my visual editor when i make posts. The default font sizes are pre-set so you cannot choose them, which is a problem for me. I am trying to add font size “13” to the dropdown options, how can i do so?

Here is an image for reference: https://imgur.com/Oqx5Ij2