I would agree that you do NOT want a 6" chimney...
The flue area should be greater than the stove outlet.
Some stoves have a 5" outlet.. that doesn't mean
you'd want a 5" flue.
I believe Harman recommends at least an 8X8 liner.
Even an 8" rnd would be less...
That isn't to say an 8" won't work. But it is small..
But a 6, is just way undersized..