For power calculations see this:-
http://www.thebox.myzen.co.uk/Tutorial/Power.html
However you need to know how much power the Triac will dissipate when it is on. Form a quick look at the data sheet I can't see the voltage between the two anodes when it is on. You need to multiply this voltage by the 5A current to get the amount of heat you will generate. The thermal resistance of the case is given so it should be plane sailing after that.
OK figure 1 will give you that power information, for 5A it looks like you are generating 5W of heat.