I use 5" and 4" feathers. The four deer I shot this year were with the 5" feathers. They clear just fine from my NAP drop-away rest. 5" feathers are probably overkill, but I'm not sure what advantage I'd get from going to smaller feathers or egads, vanes.

I've never used any type of waterproofing stuff on them. Never seen the need for it.
I do some hunting in the rain but nota ton. (you know, sugar melts in the rain.. ha hahahaha ha.)My feathers sometimes get a bit rainflattened, but thearrows still fly great out to 20 yards. (my rainy day range limit) Maybe when I do the Alaskan drop camp moose hunt I'll trythe waterproofing.

Burnie, you ever try goose feathers?