Hey Jill,
Changing the font sizes or other styles in your widgets will unfortunately require writing some custom CSS, which can either add to your existing theme or add to a child theme.
One example of changing the font sizes is to write CSS like the following:
.tribe-mini-calendar-event .list-info {
font-size: 14px !important;
line-height: 1.3 !important;
}
Here’s how that made your widget details look when I tested that on your site → https://cloudup.com/cTlWIKUwQsZ
Play around with styles like this and see what you can do. For being able to customize other styles across your site, I’d highly recommend using a tool like Firebug if you use FireFox, or the Developer Tools for either Safari or Chrome – these tools will let you see what CSS is being applied to specific elements on your page, which will make it a lot easier for you to override them from there.
I hope this helps!
Cheers,
George