@non78910,

Please try the CSS codes below:

/*use this code if Rich Editor is enabled*/
#wpdcom .ql-editor > * {
     font-size: 20px !important;
}

/*use this code if Rich Editor is not enabled*/
#wpdcom textarea{
   font-size: 20px !important;
}

/*use this code if you want to chage the font-size of the comemts in the list*/
#wpdcom .wpd-comment-text * {
   font-size: 20px !important;
}

Put the code in the Top Admin Bar > Customize > Additional CSS > "Additional CSS" textarea, save it, delete all caches and check again.

The red marked values should be changed before using. 

Please don't forget to press Ctrl+F5 (twice) on the frontend before checking.