For any brand of hood, you want the duct to be at least as large as the manufacturer recommends. Going smaller will greatly reduce the effective of the hood and increase the noise.

^ Ditto

Anything negative going larger than specified? It just vents up a foot or two then out the back.

Larger should be fine - you'll just need a transition piece to match the hood to the duct.

Short lengths may not matter either way, but just as significantly smaller ducting increases pressure loss and hence reduces cfm, larger ducting reduces flow velocity and makes it easier for fine grease particulates to condense on the ducting.

kas