Power rating, heat dissipation, thermal stability..
Fistly you would not run the resistor anywhere near its max rating, a 50% de-rating would be advisable. Secondly you start off with the max current you want to measure, take into account the voltage source and its min and max levels (using the maximum voltage to calculate max disipation). Then pick a resistor that will do the job and provide the required voltage accross it for the sensing IC.
The larger the resistor (there are special resistors out there designed to minimise thermal effects and get rid of the heat effectively) generaly the better it is to remove heat, a lot of these devices will have some sort of reccomendation for heat removal, even dictating in some instances the amound of copper the pads connectect to to help thermal disipation.
So yes the basic calculation is simple on its own but there are many other factors you employ when choosing current sensing resistors.