Look in template.css for this class
.art-postcontent, .art-postcontent li, .art-postcontent table, .art-postcontent a, .art-postcontent a:link, .art-postcontent a:visited, .art-postcontent a.visited, .art-postcontent a:hover, .art-postcontent a.hovered {
font-family: "Times New Roman",Georgia,Times,Serif;
font-size: 13px;
line-height: 175%;
}
Try commenting out the line font-size 13px by putting putting comment tags areound it like this.
/* font-size: 13px;*/
That will turn off the font size. If that fixes it, good If not you can remove the comment tags and it will go back to the way it was.
Go through your template.css file and look for anywhere the font size is specified. Try commenting and uncommenting different things and check to see what affect it has.
Be sure to save an original copy of your template.css file so you can restore it back if you make a mistake.
Ed