You could check out the links at the top of this page for information on calculating static pressure, velocity etc...
http://mikeaurelius.wordpress.com/ventilation-primer/
Just start by calculating the area of the
opening that your hood has. Then roughly figure that you want to choose a fan/blower that moves 125 cfm per sq ft of that number.
Example..
If your hood opening is 2ft x 4ft then you would require a fan that moves 1000cfm. If you have a long run of ducting with bends then you would likely require additional CFM.
The diameter of the ducting, number of bends and the total duct length from hood to exaust all will affect the performance. The longer the duct and greater number of bends will equal more static pressure.
Most fans/blowers will have CFM ratings based on a given static pressure in the specifications. If these aren't listed be careful.
For example often a fan will be rated at say "650 CFM" If it doesn't say otherwise, this is referring to "free air" as in zero static pressure. This rating will go down when the fan is actually connected to a system.
Centrifugal blowers are a good choice because the efficiency doesn't diminish as greatly with increased static pressure in comparison to axial fans.
Good luck! Take your time and choose the right fan for the job without choosing one that's overkill.