Depends on the power system you are using.  Medallion .049, probably 42'  Tee Dee on pressure with a drilled out venturi 50'  PAW .061, I expect you could go to 52'. 

Thanx Larry & Jim.  I'm planning on an UNSPEAKABLE power system but in the spirit of 1/2A I will limit it to a 6" prop.

How about total flight time and/or lap times to get a full pattern?  I am assuming my usual 5.3 sec laps arent gonna work...

Hummmmmm the smell of electrons are in the air.
Dennis

The smell of electrons are OK but the smell of the black smoke when things go wrong is hideous!

Denny, I don't have a lot of experience with exactly what you are up against but as long as you are not tied to competition rules where you must tailor the aircraft to a specific length of lines, finding what works best for this  airplane  is going to be like choosing the best prop for a certain plane motor combo.
I think you will need to make up several sets of different length lines and try them all.

One thing that has not been brought up and needs to be considered is that just two feet can make a very noticeable difference. This is especially true at longer line lengths. What I am trying to say is that I would not go right from 42' to 52' in one step.
In fine tuning this wild beast (which has my attention!) You might find that you need to tweak the engine offset or rudder to get it to fly on the 52's. Who knows, you might find you like it best on 50' lines.

It also sounds like you might have propeller issues as well if the motor is as you say. If you know without a doubt you have a large surplus of power, there has to be a level of inefficacy somewhere. I am sure you will be able to wade though this and have a massively rewarding experience!

I look forward to seeing more about this project, Robert

Hmmm,
as well as the 1/2A P'finder flies with a Norvel .061, "IF" you are going to electreic, I am also "very interested".
Please keep us advised.
Roger V.

IMHO, Spiderwire works, but .008s have less drag.  Line weight doesn't seem to be an issue.  Of course the Spiderwire is more durable.

I was using 35 foot lines on my electrified Baby Clown. That was more or less set by the battery (a 2s730mAHr pack) and ~2400kV motor that I had wound myself.

Here is a link.

http://stunthanger.com/smf/index.php?topic=8841.0

well my 1/2 A is a little more powerfull than most, Cyclon .049 and the model is 315 squares. but the last times i flew it i was using 60 feet of .008's .i will probably cut them down in the future since i finally got the speed down by using a 1.2 pitch prop.since the lap times were a bit slow the lines dont need to be that long.allthough the model flew fine with the 60's it takes a lot of fuel to do the pattern at the long lap times.and the line weight becomes excessive.
Dave jr.