I believe it's 3/8". There are a number of them, from a few dollars to 10 or 15. You put the tool around the male end, and slide it toward the female end, which pushes the tabs in to release it. Give it a tug, and the line should come right out. You'll need to replace the O rings too. A search of "quick disconnect fuel line" should get you the part numbers.
Don't forget to bleed the pressure off the line before you do that- the schrader valve there on the rail.