Thimble Size?

I just bought an Energy King 480EK furnace; the furnace flue size is 6". Now its time to build the chimney. I'm planning on building an outside masonry chimney with 8" square flue. Should I go with a 6" thimble to match the furnace flu size or should I go larger to an 8" thimble and use a 6" thimble to reduce the size? I was thinking of going 8" for future use, but then again how often am I going to replace the furnace.

Thanks,
Dave

go with 8" never know what you'll need in the future. The 6" pipe will fit right inside the 8" thimble