We use dacron line typically sold with a 1/2a handle at the hobby shop. Usually there is 60 to 70 feet of dacron line on the spool which will result in lines about 30 to 70 feet long. A good starting point is 30 ft, which will work well in most cases. You can adjust the line length for the plane and engine combination and resulting line tension. For a slower plane use 28 ft or so and for faster planes try 35 ft.

Kevlar fishing line works well but is expensive. Typically sold as Spyder wire at Wally World.

We fly 1/2a all summer long and rarely break a line using the dacron stuff. Not one break all last summer! It's very cheap and strong. Less than $1.00 for a spool, change it when it gets frayed or really dirty. You can buy it from Sig, Brodak, or Tower Hobbies if not available from your local hobby store. Send me a PM and I'll send you some.

Braided cable can be used but is very fragile and much more expensive. Use .008 for 1/2a. I've never tried .012 for 1/2a, I think it is way too heavy for the typical cox motor to haul around. Probably OK for a 1/2a plane with a Norvel on the snout. Norvel engines have much more power. .012 cable is usually used with planes powered with .09 to .19 size engines as I recall.

Don't spend too much time worrying about it! Get some string and have fun!