There's a simple thing to try first. Find and open the file includes/templates/YOUR-TEMPLATE/stylesheet.css
Near the top of that file, find this...
Code:
body {
margin: 0;
font-family: verdana, arial, helvetica, sans-serif;
font-size: 65%;
color: #000000;
background-color: #e5edf5;
}
Change the red part to a larger percentage, perhaps 75% or even 80%, and save the file to your server. That will increase the base size of all text on the site. Look through your site and see if you like the result.
Once you've got that base size set the way you want it, then you can adjust specific bits of text independantly elsewhere in the stylesheet. For help with that, we'd need a link to your site.
Rob