I use 3/16 for my brake lines as there is not much flow required on the brake system. I think thats pretty common. On the clutch side I used 1/4 because of the higher flow volume and speed through the clutch system. I wanted as little resistance as possible. Double flare everything unless you are using stainless line. Stainless doesnt like to be double flared as it tends to crack. People probably have different preferences but this worked for me. Have fun!
Blaine
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