Basically you can move any amount of gas thru any size duct up to a point of course. BUT as the duct gets smaller the gas must move faster and will require more force or pressure to move it. It will also generate more noise as the duct gets smaller. Short answer is to use the largest duct possible that satisfies the other criteria; fan size and power, tolerable noise, physical constraints, cost, etc. In your case the 6" ducting on a typical range hood would be fine and the cost/availability factor is large.
PJ
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