At 6'6" anything in "tall" is my usual wardrobe.
BUT
Why get the bottom of your vest wet when wading?
If you really want a long vest, look into a photographer's vest.
Otherwise, I would stay short.
Personally, I use a waterproof waist pack that gets dunked regularly.
Oh, and I do frequently wade in water deeper than my waist. Probably not the best idea.