Oh, I figured out how to change the font size in the product description. On my stylesheet.css I added:

font-size:90%;

to product description so it looks like this:

#productDescription {
border:1px solid #bfbfbf;
margin-top:20px;
clear:both;
font-size:90%;
}

Now I want to figure out how to change the size of the text in the sidebar.