8 and 1/2 feet, at least. We use 8'2" Boss's, and I would like them to be wider, ie. 8'6". Get a vee plow of any brand (except Meyer), and enjoy. Your truck won't mind the size or weight, and your pocketbook will appreciate the productivity.
Regarding the lift, the key snow plow aspect to respect is that the A-fame of the truck should be level when the blade is on the ground. You would probably have to have some customization done to your mounting kit, because I don't think there is that much adjustment in any plow kit available. Don't be tempted to modify the blade itself -- modify the frame kit. Kits can come and go, but you may transfer the blade to other vehicles in the future.
I would put a 8ft with a straight blade, 8.5 for v blade.I would not plow with a truck with a 6inch lift, it is to much stress on the u joints. A 3 inch would be the max.also the higher the truck less Visibilty you will have, the harder to plow and more apt to hit things.
First, if your going to install a plow on your truck, make sure you have the X code front springs installed. Ecspecially if you have a Powerstroke like mine, but you should do so even if you have a gasser.
Like everyone else has said, you need to get at least an 8 1/2. I have a Western 8 1/2 V and on a sharp corner my rear wheels are outside the plowed area (mines a Supercab).
The truck handles the plow very well. My plow wieghs 880#.
I do think it will be hard to see though with a 6" lift. My truck is about 2 1/2" higher than stock and it is hard to see the ends of the plow.
Solid advice so far, go with an 8.5 ft. About the lift... you will probably need to lower the mount on the truck. Being up that high might cause some visibility problems in tight areas but if you are used to driving lifted trucks that should be ok. I would imagine with a 6 in lift your probably running some wide mud tires right? I will just say that the skinnier the better .
Just for you own info I used to plow with a K-5 blazer that had a 4 in suspention lift and a 2 in body lift it had 35x12.50 BFG all terrains. I soon put 31x9.50's because it skated all over. I also ended up taking the 4 in lift off and put on a 2 in lift . With the lift I never compensated by lowering the truck mount so when the plow was at full andle the corner would not touch the ground. It is important to have the plow be parallel to the ground.