Do a search on flying lines.  Myself I use .012 cable for 1/2A about 35 to 40 feet in lenth.  Most guys are using what is called Spider Wire, available from your local bait and tackle store, maybe even Wally World.  Maybe Minnesota Modeler will chime in here with the real deal.

Myself, i use 30lb. spiderwire. I have been using it for several years, trust me on this, if you are just trying to tie the line to a connector or swivel 3 overhand knots will hold. Yes if you decided to try some fancy smancy knot it may fail, especially if you are trying to tie it in a loop and hold it there without slipping, and i have used palomar knots successfully, but this is 1/2a, i have used and will continue to use 3 overhand knots. By fail i mean slip, 30 lb. spiderwire is way strong and small enough to give feel and feedback on 1/2a's. I use fishing swivels on the end of my lines for 1/2a's  and after 3 overhands and pulled tight i cannot make one slip! As far as line length, it will totally depend on the power output of your engine and size of ship. Mostly on Cox reedies i will use 32 to 35 foot lines. I have a flatwinger that flies very well on 35 foot lines with a medallion .049 and balloon tank. Built up airfoils will do better. I have a "bunch" of norvel powered ships that i fly on 45 to 50 foot lines, depending on the wing shape and weight. I tried .008 steel wire a long time ago and gave up on that idea because of kinks and trouble making good ties to the grommet type ends because the line was so thin. It's kinda like my golf teaching pro told me a few years ago, "find what works for you and do it"!!