Usually fuels ines are 5/16" and like firebird said if you are running big HP, 3/8 is a better choice.
As far as brake lines go you can use all 3/16" and not have any problems. I plumbed my 50 wagon with all 3/16" and it has 4 wheel disc. The big thing with running discs/drums is making sure you run a 2 lb residual valve for the front discs and a proportioning valve.Preferably an adjustable one too. You also have to figure out what type lines you are going to use. Hard lines will use a double flare connection whereas if you use any braided steel flex lines then you are getting into 37deg flares(AN fittings).
All the hard lines and T-fittings you can get at your local parts house and they may even have the AN stuff. But if not then Jegs or Summit carries everything.