The font size for the body text on that site is 40px on my monitor and 35px on the laptop screen. That would be large for top-level headers. 16-18px Arial is just about the perfect size for primary text on any screen. It was a little excessive on 800x600 CRTs in the '90s, but not by much.

There are some nicer fonts than Arial/Helvetica out there, and I'd say the author used one of them here. The text looks really good at 18-20px, which is about the same size as 16-18px Arial.

I can't understand how so many "experts" are constantly screwing this up so badly. Some of the worst offenders are web design and typography bloggers.