Flue pipe size -- will 6" work in my mass heater?

I am planning to build a mass heater in my fur shed. I currently have 6” flue pipe for my woodstove. So my question is can I use 6” pipe to build my mass heater or do I need to use 8” pipe. Seems like everything I read or see says use 8”.  Which is best?

Using the existing 6" exit pipe requires that you either build a 6" system or replace the existing exit pipe with an 8" one if you wish to build an 8" system so the cross section area can remain the same.

Jonathan is correct. For a more specific answer applicable to your situation, we would need to know things like your climate, the size and character of your fur shed, how warm it needs to be and for how long...

If you need a lot of heat and you have an existing 6" chimney, you could build a batch box combustion core, which dumps more heat faster than a standard J-tube of the same size, so there is that variable to consider also.