Well, no easy answer as nobody knows what geometry, material, environment your system is operating in.
First thing you need is the a rough estimate of the airflow needed to dissipate a given amount of heat at say sea level. It should be noted that the mass of air, not its volume, governs the amount of cooling.
If using an off-the-shelf heat sink there is usually specifications on which fan or other you need. If not the following is a guide but not the definitive answers on what you might need.
Establish Cooling Requirements
Before a fan and the CFM specified, the airflow required to dissipate the heat generated has to be approximated. Both the amount of heat to be dissipated and the density of the air/fluid must be known.
The basic heat transfer equation is:
q= Cp x W x DT
q = amount of heat transferred
Cp = specific heat of air
DT = temperature rise within the system
W = mass flow
Mass flow is defined as:
W = CFM x Density
By incorporating conversion factors and specific heat and density for sea level are, the heat dissipation equation is arrived at:
CFM = 3.16 x Watts / DT (°F)
This yields a rough estimate of the airflow needed to dissipate a given amount of heat at sea level. It should be noted that the mass of air, not its volume, governs the amount of cooling.
Determining System Impedance
After the airflow has been determined, the amount of resistance to it must be found. This resistance to flow is referred to as system impedance and is expressed in static pressure as a function of flow in CFM. A typical system impedance curve, in most electronic equipment, follows what is called the "square law," which means that static pressure changes as a square function of changes in the CFM. For most forced air cooling application, the system curve is calculated by:
P = KrQn
P = static pressure
K = load factor
r = Fluid Density
Q = Flow
n = constant; Let n=2; approximating a turbulent system.