Ok, here is a very simple answer to a very simple question.
Inside of your website admin area under appearance is a place to put custom CSS called Edit CSS.
At the bottom of whatever else is in there add the following code:
#bbpress-forums .bbp-topic-content p, #bbpress-forums .bbp-reply-content p {font-size: 16px;}
That will make the font 16 pt. if you want it some other size then change the number.