Probably you need to create a shortcode with parameters. You should have different parameters for font size, color, bold, underline or anything else. The user should insert a value for each of these parameters in the shortcode.
You can find what you need here
https://developer.wordpress.org/plugins/shortcodes/shortcodes-with-parameters/
It is a little tricky though because you must check the user input, because you can never be sure the user will input the proper value of a parameter.