Edit the style.css file for the theme you are using. Editing via FTP, if you are using the default theme, the path to the file would be: ndxzsite/default/style.css.
You can also edit the style.css file in the Indexhibit admin area by clicking the Assets tab, and under the Template section click style.css.
Look for the following section of code:
/* styling the index */
#index ul { list-style: none; margin: 0; }
#index ul.section { margin-bottom: 1em; }
#index ul.subsection { }
Change the code to look like this:
/* styling the index */
#index ul { list-style: none; margin: 0; font-size: 26px; }
#index ul.section { margin-bottom: 1em; }
#index ul.subsection { }
Note the added code: font-size: 26px;
This will double the size of the section titles and subtitles. You can play with the 26px number to suit your needs.
PS it is a good idea to duplicate the default theme folder and give it your own name. Then select your duplicated theme in the admin area. This way if anything gets messed up you can revert to the default theme and start over.