for a large broadhead I haven't found anything that will control it better than a 5 inch feather.

4" feathersseem to stabilize my broadheads just fine, and I release with fingers andshoot fixed blades. I would use 5" for an even great fudge factor since, according to the chrono,the extra drag of a 5" doesn't affect speed at short range. I've never taken a shot over 30 yards, and probably won't where I hunt, so it's all the same to me. I went to 4" because I wanted a bit more clearance for my fingers and moved my fletch forward a bit on the shaft, and my brace height adn restruled out the 5".

Longer and helical isalways more stable than other other combo.