The Big Mig should be good up to about 12 ounces, then you have to start going to shorter lines. At 12 oz. with a 5.5x2 APC prop or Cox 5x3, I would think you could fly 45 ft x .008 lines with ease.
If weight gets up to 14 -15 ounces, you are no longer going to be competitive, but will be safe and have fun on 40' lines.