RE: Vane size for 125 gr fixed heads?
I shoot 125 Magnus Stingers on 29" GT 7595 with 4" vanes. never had an issue with getting good flight. Before that, I shot 100 gr. Muzzy 3 blade, same vanes. No issue there either. Work with your bow for good flight, buy good broadheads, and you won't have a problem.
JMAC