There's a faq about vent hoods on this forum. There's a link to all the faqs near the top of the appliance forum page. I've attached a link below to the vent hood faq.

The rule of thumb is that you add up the BTUs for all your burners and then divide by 100. So if the total of your cooktop has 60,000 BTUs then you should get a 600 CFM hood. This is a ballpark figure since in general you won't be using all burners at maximum. For most applications a 600 CFM hood will be sufficient. But if you have a grill for example you might need 1200 CFM.

To search, go to the bottom of the appliance forum page and do the search there - you have a choice of searching just this forum or the entire ths site.

Here is a link that might be useful: Vent hood faq