Wood stove flue hole size

I have 6in flue pipe that is double walled. Do I cut the hole in the ceiling and roof at 8in?

Check the manufacturer.  Much depends upon what the ceiling and roof are made of.  I have seen recommendations of  6 inches of clearance for combustible material.  Braver people than I have used far less

From what I am aware, double wall pipe requires at least 6 inch clearance from combustible materials. most commonly used is triple wall pipe where it goes through ceilings and roofs, which requires 2 inch clearance.
most 6 inch (inside diameter) pipe has an 8 inch outside diameter , add 2 inches on each side for a total hole diameter of 12 inches.
For double wall add  12 inches  (2 X 6)  to the outside diameter. This large of a hole can be more complicated / expensive to seal / flashing on the outside
I hope this helps

Thank you fir the response's I might have to call the wood stove store and see what they recommend. I did draw out a hole that is 12in but not cutting it till I know exactly what size it should be. I don't have the part that the flue goes through at the ceiling nor the roof yet and my suspicion is it won't be cheap..

What do you mean by double walled pipe? Big fat insulated or skinny stainless/black?

If it's a house/residence, get the thru-the-wall/ceiling kit from the same manufacturer. These are certified, but only as a complete system. If you improvise, insurance will laugh at you and deny coverage.

Whether you're in a home where you are going totally safe, or a shop where you might just be getting a pipe through the roof quick and dirty, you should be using something to transition (rather than just a round hole in the ceiling.)

They make fancy transitions called thimbles for homes that interface with your double or triple wall pipes. This is the safest way to go.  

If you're just getting by, get a flashing boot. Here's a picture of mine that I installed just this weekend. You set it on the roof and trace the square, then draw a square about 2 inches smaller inside. The smaller square is what you cut out. These boots come in a size range. For my 8 inch pipe, I ordered a boot that accommodates pipes 6" to 11". And going larger on the size range like that helped with the steeper angle of my roof. Then I just cut the silicone boot a bit smaller than my pipe diameter and stuck it through from below.

This is the style roof Jack that is common around here. Cost is around $70 A collar slips over your pipe from the top to seal the hole.
They make a second Jack if you have a flat drop ceiling that is a bit more.    

I have a used roof jack but don't have a heat shield that goes between the double walled stainless pipe that is in the attic and on the roof. I also don't have a heat shield that goes between the hole in the ceiling to the attic. I have a sheet metal box I might be able to use but need something to hold the the tip of the double wall black pipe that is coming from the stove itself..this is a telescope pipe to make it easier to install. I don't have a lot of money and everything I read says the heat shields both for the ceiling and roof need to be insulated between the house framing and the flue pipe.

How do you plan to support the weight of the double-wall insulated pipe? This can be quite substantial. It can't sit on the pipe from the stove.

Wall/ceiling kits include a cradle to support the insulated pipe, as well as a twist-lock connector that mates the end of the insulated pipe to the double-wall pipe from the stove.

I have used improvised setups in outbuildings, running single-wall pipe through an insulated pipe in a window. But except for emergencies I don't think an improvised setup is good policy in a house. Fire is a two-faced friend. My 2c.

Yes I was aware that the flue pipe needs to be held up in the attic and the sheet metal box although good enough to use it's missing the ring that holds the pipe in place and no edge to sit inside the attic above the ceiling.
I'm sure I can use the roof jack for sure but as you stated its not exactly safe to improvise when it comes to my house starting on fire. I'll go into town tomorrow and buy the parts I need to do it safely. Thank you for all your help. I love all the people here on permies.  

Went into town and asked and I will need a few parts to make it work. I have another question..one of the 18in sections of flue pipe is bent inwards at the screw bottom so its not round, is there away to fix this?

It it's a small bend, it may be possible to bend it back using vise-grips or large pump pliers.

A picture would be helpful. There are lots of people here who know how to work metal.

Here is the damaged section