I am installing a Fluidmaster Fill valve.. I have a metal flanged water supply line. Instructions say to use the existing coupling nut (have to.. Can't get it off anyway) and the existing washer. If this existing washer is a flat washer that sits on top of the flange, there wasn't one when I disconnected it. When I hooked it up and tightened the coupling nut, with a wrench, as tight as I could,there is a leak around the coupling nut. I'm assuming I need a flat washer to sit on top of the flange.. What size/type of washer do I need? Or is there something else that needs to be done?

OK! Here's a toilet supply, (see image) They're all the same. The small nut will connect to the outlet of them angle stop and the large nut should have a plastic built right into the nut. If you don't have that washer return it and get another.
Now for your leak. If the supply you installed doesn't have a washer you have found your problem. Replace the supply. But there's another reason you could have a leak. It's very easy to cross thread plastic threads. It will tighten down but leaves a tiny opening that will leak. If your supply has a washer then take it apart and try installing it again. Good luck, Tom

Thanks for responding.. The issue is that I do not have this type of supply line.. I have a flanged metal tube.. There is suppoded to be a washer that sits on ope of the flange, which then seals against the threaded shank of the fill valve assembly, as the brass coupling nut is tightened... Fluidmaster instructions said to use the "exisiting" washer from the old hookup with this type of water supply line. My old one had either disintigrated or was not originally installed. Fluidmaster illustrates this washer as a "flat" washer. My question is, "what size/type flat washer do I need??" Thanks..

Is this what you have? (see image).
Let me know, Tom

Yes! That is the tube I have.. Thanks

The yellow insert is the washer. But wait! You're doing it the hard way. Take it back and pick up a supply O have put up in post #2.

Much easier to work with. Good luck, Tom

Yup! Even if you were successful in getting the right washer (could just buy the smallest metal toilet supply and swap the washer) the other parts (nut/ferrule) have been compromised and will probably cause trouble for you here.

As Tom said, buy a new flexible supply... has rubber cone washer inside the nuts at both ends of the toilet supply! Tighten each nut end hand tight plus a 1/2 turn or so and then test your work.

Good luck!

Mark