Weeeelllllll----I won't tell you that I flew a Magnum for two years that weighed in at a robust (spell that anvil) 72 ounces with a Merco 61! It flew better than I did at the time - 1992 & '93 and, as with all heavy ships, there are no small flying errors. They always compounded to more than small for obvious reasons.
With the aerodynamics of the Magnum I think anything in the lower 60 ounce range would be a really top notch stunter. (60 to 65 oz.) 'Course, I've been wrong before!
What power are you going to use? I really loved the Merco, and I understand it is out in Oregon now still stunting like crazy!
Blessings,
Will