you know the ten-gallon hats? I've never understood how the unit of measurement applies to the hat - ten gallons of what? Hat? Can you explain?

no.

I can only tie it to the policeman hat part, so maybe a ten gallon hat means it can take 10 gallons of pee? That's a lot of pregnant women helped out. Or maybe I am wrong..

actually I'm lying. It's either from the Spanish for "really fine" or it's from the Spanish "gallon" style. Either way it's Spanish, will that do?

How can a hat be in the style of a gallon?

dunno, ask the Shapish. That right there is an I've-just-walked-in-the-door-and-I'm-very-hot typo for "Spanish".