The fuel line for my 1980 Mercury 200 HP is 5/16. I confirmed that when I installed an in-line fuel filter a year ago. Interestingly, when I bought the boat 3 years ago, the only fuel filter was the screen which is on the outboard.
I added a fuel-water separator also, which was 3/8", so I ended up buying this reducer to transition from 3/8 to 5/16.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1