Although there are many factors that will determine what size exhaust fan you need, as a general rule, the following will apply:
For under cabinet range hoods or hoods mounted against a wall the minimum air flow (CFM) is 40 CFM per linear foot.
So a 36" range hood should provide a minimum of 120 CFM of exhaust. This is a minimum.
The recommended amount is 100 CFM per linear foot. For a range hood mounted from the ceiling (island mounted range hood), the rates are higher.
A minimum of 50 CFM and a recommended rate of 150 CFM per linear foot
NOTE: Always consult any applicable building codes in your area in regards to minimum and maximum air flow rates. Certian states may require additional items such as make-up air for larger CFM range hoods (typically over 300 CFM).