Compensate for longer duct run by using larger diameter?

I've installed a wood stove type fireplace (Quadrafire Pioneer III). It has a fan that will shove heated air out the top front; the supply of that air can come from a duct run to another location in the house. (Combustion air is supplied by a separate duct run to an outside wall.) The fireplace manual says to run a 6" duct a maximum of 40' to a register to supply return air to the fireplace/stove heating system and fan. The ideal duct location for me would require a 60' run. My thought is to increase the duct size to 8" to compensate for the longer run. I had an HVAC company out helping figure this out and they said a larger diameter duct could be problematic as it could reduce the efficiency of the fan (??). They pointed out a larger duct would result in lower velocity of air flow which I agree with. But I don't think air velocity is an issue here (like it might be for a heat exchanger).
Is there a rule of thumb re duct size vs distance that would suggest what duct size would allow me to go 1.5x farther than would be OK with a 6" duct?
Is there any reason using larger duct would be problematic?

source https://hvac-talk.com/vbb/threads/2228254-Compensate-for-longer-duct-run-by-using-larger-diameter?goto=newpost