8-inch will have lower pressure loss than 6-inch so there should be some slight improvement in blower performance. The limitation on duct size is that if it is so big that the air velocity falls much below 1000 ft/min when the cooking requires the full blower performance (hot greasy cooking), grease particle condensation in the duct may increase.

You should be able to get 400 - 500 CFM out of your hood, depending on baffle characteristics, assuming decent MUA performance. At the low end of this estimate, the duct velocity would be 1146 ft/min, so 8-inch duct is fine. You can use a 6 to 8 transition duct to minimize pressure loss from an abrupt change in diameter.

Thanks so much for the fast response and detailed explanation!

To understand better, are you saying a 8” duct actually is better here? Or would a 6” duct be better, but 8” is fine? (Or the two will basically be the same?)

Also, when you’re estimating air velocity with respect to grease particle condensation, does that become a concern if the hood isn’t being run at full speed? Or is the lesser amount of grease particles being produced in such cases not something to worry about?

Good info from @kaseki. You want a long transition, like at least 12" (1.5x the diameter) when going from 6" to 8" in this application. The shorter the transition the greater the eddy currents and so grease build-up that would be a fire risk.

You understand that that hood, no matter how many CFM's, will perform quite poorly and likely be quite loud? It will be great for boiling water on an induction but not something you'd want for anything else.

Yes, I meant 8-inch will be better, in general.

Grease collection at various points is a complex matter. For example, one reason simmering bacon, in spite of its low upward plume velocity, needs a higher than minimum air flow is that the baffle centrifugal extraction process needs enough velocity to work. In general, however, hotter cooking generates faster plumes and needs higher hood intake air velocities. If the duct is warm enough and the flow rate not too low, the grease collection via condensation will be low. If the velocity in the duct is not too high, the grease collection via impingement will be low. Most home cooking levels are such that what does collect in the duct rancidifies (hardens to a varnish) and doesn't become a nuisance or fire hazard. Commercial ducts, however, need yearly cleaning and inspection.

If performance is your priority, then review of hood threads here will provide insights into the requirements needed to achieve good capture and containment.