One way you can do so is looking at your panel(the main circuit bakers) to find out how many amp's it is, maybe about 100 amp or so. then take a look at your power bill.
the power company charges you by the kWh, figure in the total kWh per-day and that is you daily usage.
Over sizing the generator for more kW is good for those appliances/tools that can take up more power.
But this sizing of the generator/math should be based on the max power the house can use, this is so you can use other tools that say a contractor may use.
Also keep in mined that a residential home has 2 phases to power a range/oven, water heater, dryer, A/C unit, Heater. so you would have to ether buy a 2 phase generator or get 2 generators 1 phase.