The longest legal line length per AMA is 70 feet. Add an arm's length and a couple of awkward steps and you're at 75 feet or so total length. We are working on a new site right now, and figuring 200 foot diameter circles with 20 or 30 feet of buffer room between two circles.

FWIW, if there are power lines in the area, AMA suggests 50 feet clearance. Most flyers prefer 200-300 feet from high power lines. They can arc over under the right conditions. Kinda ruins your day to get kilo-volted

Our last field had 5 foot diameter concrete pads in the center to give flyers a reference for staying in the middle. I really like these 'cause I tend to wander...

Hope that's some help.