You want to increase the size of this? See https://i.imgur.com/vsJZGYr.png
This increases it by 25%:
ul.page-numbers {
font-size: 125%;
}
To add or override CSS: use the “Additional CSS” option in the customizer. https://codex.wordpress.org/CSS#Custom_CSS_in_WordPress
Learn to use the Chrome Developer Tools or Firefox Developer Tools to help you see and test changes to your CSS.