If you mean the brake plumbing (steel tubing), it's 5/16"...I just re-did mine.
However, I'd have to agree with oldfamilyguy on this, I wouldn't trust the guy to change the line. He has the vehicle and couldn't just take a quick look for himself? Unless the lines are seriously jacked up causing it to be unsafe to drive home, you should do it yourself. All you need to do is measure the length required and have the parts store cut it for you. While you're there, pick up a quart of DOT3, some new fittings and borrow a hose flare tool. If you're not sure, you can always pull the line you want to replace and do it all at the parts store. Then all you have to do is re-attach the line, then flush and bleed the system. <----not optional