This site is using a font-family named Avenir Next its not a Google Font though, so you need to find this on a Font Library site and integrate it on your site. You can upload a custom font on the Font Manager with
.woff2 file format, and use that throughout your site.
To inscrease the general font-size on your site, please navigate to Theme Options > Typography and adjust the Root Font Size for FONT SIZE (XL) and FONT SIZE (LG)
Please read these posts, those will help you understand how Font Manager works.
Introduction to the Font Manager