I can tell from your screenshot that this is the Classic Designer.  However, this is one of the few cases where this code will work on both the Classic Designer and the Modern Designer (I tested both on Version 11.0.2307.40547)

CSS:

/*This is if you want it applied to all fields.
Note that 1em is equivalent to 16px, so these function the same way,
you will only need one of them.  
Using em adjusts to different devices better than px does.*/
.cf-helptext {font-size: 1em!important;}
.cf-helptext {font-size: 16px!important;}

/*This is if you want it applied to a specific field, you can use
text like this, that only refers to fields that you have added
the largeHelpText class to.  Once again, the two lines
are effectively the same, and you only need one of them.*/
.largeHelpText .cf-helptext {font-size: 1em!important;}
.largeHelpText .cf-helptext {font-size: 16px!important;}