From the information you have given, you are obviously talking about an industrial type of device. Consequently you will need to consider other things such as an automatic cross over switch and other items.
The units of measure will generally be in KVA. Note that the K means “thousands”, the V stands for voltage which you have specified, and the A stands for current in Amps. If you already know what devices must have power, they will likely be part or all of a substation near your switch gear (breakers, etc.). Frequently you can observe the typical current being used at that point. Understand, however, that many of the devices, lights, and more, may not be in use at the time of your observation.
In order to leave a little room for the unexpected, you need to consider the maximum current needed at any point right now, add 10% for future growth and make sure that figure does not exceed 80% of your generator. If you run above 80% of the full load power, you will generally wear out the generator before it should be. Remember that some loads require a momentary higher than normal current to start up. That has to come from somewhere!