Capacitor as one piece voltage controller for wind turbine
The alternator generates higher voltages but will require over speed of the generator. 


You probably mean to say that you need overspeed regulation to prevent overspeed.


  I'm looking to custom design a capacitor for a complete one piece regulator for this.


You are extremely unlikely to be able to build a better capacitor for 380 V AC in any reasonable capacitance than is commercially available.


  How do I determine the heat dissipation (in watts) of a capacitor.


Ideal capacitors do not dissipate any power.

Real capacitors have some internal resistance and the "equivalent series resistance" (ESR) value gives you this. Power dissipation will be given by \$ P = I^2 R \$. I for a capacitor is frequency dependent and is given by \$ I = \frac {V}{2 \pi f C} \$. (These are RMS values.) Using these two equations you can work out the power. Note that the capacitor will have a derating based on frequency due to dielectric heating. This information will be quoted on power-factor correction capacitors which are designed to be connected directly across two phases.


  I would like to use a very large capacitor to stall the turbine above 380V (to prevent over speed) ... 


It is unlikely that you want to stall (stop) the turbine. It is more likely that you want to increase the load to prevent overspeed.


  ... and to divert extra power to heat. 


If you want to generate heat then you need resistors.


  I've been looking at some Air Capacitors but generally those are very high voltage. I'm really needing a cap that is 380v - 480v ...


Have a look at datasheets for power factor correction capacitors.


  I can do this with some smaller capacitors but the heat builds up and they just can't handle it. 


Either the ESR is too high or the dielectric is the wrong type for AC at that voltage.


  I've tried some larger capacitors too ...


It sounds as though you are using trial and error rather than electrical engineering principles.


  ... but I'm just needing some help calculating maximum voltage clamping capabilities.


Capacitors do not clamp voltages. On DC they store charge. On AC they will behave as an impedance and pass current. The voltage and current will be very close to 90° out of phase and so little heat will be generated.

From your comments:


  But the problem I am having is over heating and I am trying to properly size a capacitor for the task of using it as a Zener Effect.


Capacitors do not have a Zener effect.

Note: 'V' for volt. 'W' for watt. 'A' for amp.