There are two options if you or a user wants to increase the font size.
The simplest option is that each user can simply zoom in or out as required. dbFront will automatically adjust the site layout to accommodate your tables and forms.
If you as an administrator want to change the default font size then you will need to augment the CSS for the specific theme by adding your own custom CSS file.
The first step would be to add a custom CSS file. Adding a custom CSS file is described in: Where should I add Custom CSS.
Once you have a custom CSS file then you can change the base font size by adding the following CSS. The following will override the site default font size of 0.7em.