You can not use this hose https://www.amazon.com/Flexzilla-HFZ38100YW2-inches-feet-Hose/dp/B0010X7J78/ref=redir_mobile_desktop?ie=UTF8&psc=1&ref=ppx_pop_mob_b_asin_title on that pressure washer outlet.

The size of the snap fittings on the ball valve and gauge are 3/8"

There are 2 common sizes: 1/4" (quick connect nozzles go in the 1/4" snap fittings) and 3/8" usually for hoses.