What size would fit someone with 7 7/8 hat size?

My cap size is also 7 7/8...big by any standard. I bought the XXL. It does fit but it's snug with hair. If you shave your head, it would probably be fine. I should have gone to the XXXL.

my head is about 22 4/5 inchs,can i wear it?

Good question. Quik's size chart shows 59 cm for a Large/XLarge (23.22 inches). You should be good to go!

This hat is not water proof but I wore it all summer to the beach/pool IN and out of the water. I never took it off. It faded some from the salt and sun but still holds up after playing in the surf. I have an O'Neal hat that is the same shape but the material feels more synthetic (Quicksilver is more canvas/cotton like) and the O'Neal color is still black. The O'Neal also sheds water better but is also not water proof. I like the Quicksilver better because the rim is sligtly larger giving me a bit more sun protection.

If i wear a 7 1/4” typical baseball hat, what size should i order for this hat, the l/xl size?

It's unfortunate this manufacturer isn't more specific in its sizing information. My head measures 23" in circumference. The large/extra large fits well - not tight. Fine hat. I would definitely suggest the large/extra large for you. You can always snug it up a bit if its loose, but you can't enlarge it.