There is probably a new thread every week about sizing generators. The rule of thumb is always "Size it to the loads it will be servicing."
Manufacturer's have software that takes the loads that you tell them and calculates how many load steps would be needed for Transfering from normal power (and the minimum size generator needed).
For supplying normal power you need to oversize (depending) for motor starting. If you have a large motor being used for ventilation or a compressor you need to make sure you can start them while the generator is under full load. Once again, if you are unsure about this, talk to the generator manufacturer and they will plug the numbers into their software and determine what the minimum size generator you will need.
You could always ask the Generator Manufacturer if you could get a copy of their calculation software. It might even just be a glorified excel sheet with Generator stability equations. You might be able to find some generic stability software that would require generator information that a manufacturer may or may not provide. Or, you could learn how to use MATLAB/Excel and do your own calculations; in fact, I did this in college to make sure a Generator controller was stable before it was implemented.
I generally oversize equipment and not put motors on the standby GenSets (except for the HVACs cooling network and server equipment). But, I am an end user and don't have to bid for contracts.
