How do I increase the font size of the hover text on my form? It's appears very small.

You have to inject the following custom CSS codes in the form:

.form-description-content {

    font-size: 18px;

}

Change the value to your preferred size. 

Here's how to inject: https://www.jotform.com/help/117-How-to-Inject-Custom-CSS-Codes.