.15-.19 engine use 0.012" diameter lines 52.5' long. .35-.40, 0.015" diameter 60' long. The line lengths are tradional and probably were derived from the speed and racing events. 7 laps on 60' is exactly 1/2 mile as is 8 laps on 52.5'. Makes the math easier to calculate speeds. I have not flown any half A since I was a kid.
Chuck