use a CSS Insert (ATO->Add HTML/CSS Inserts->CSS Inserts) with a CSS selector identifying the widget. then you can use the CSS 'font-size' property (do a google search using 'css font size' to find the options)
You will need to look at the source of the generated page to identify the widget - it will look something like this
HTML Code:
<div id="text-2" class="widget widget_text">
<div class="widget-title">
<h3>your text title</h3>
</div>
<div class="textwidget">your text is here</div>
</div>
in this case you could make your selector '#text-2 .textwidget {....}'