The main body font size can be changed by adding this small CSS snippet at ‘Appearance’ > ‘Customize’ > ‘Additional CSS’:
body {
font-size: 15px;
}15px is the default size. Change this to a different value.
Some parts of the theme (headings, navigation etc) have their own defined font size.
If you want to change the size of a specific element, please let me know exactly which you want to change, and I’ll be able to assist further.