Richard:
I agree with John, just put down as many as you can and don't worry about the gage of wire and lengths.
I have 25 25 foot radials underneath my 43 footer and I just have them laid on the ground, pinned at the ends with cabob wires. They're about 8 to 10 inches long.
The grass is starting to cover up the radials. I have them laid out in a pie shape with a quarter of it taken out because my cottage falls in that line.
I have the Zero Five 160/10M fold over model.
73:
Mike VO1AX