When sizing a powerplant for a GA aircraft, how do you estimate the power required for other electrical systems?
This depends heavily on how the plane is set up especially these days with the prevalence of glass in the cockpit even in GA planes. For a point of reference most small GA planes like the Piper Cherokee run 60-100 amp alternators which usually have a bit of headroom in them but power everything and charge the battery just fine. If you want to fly in most airspace you are going to need to power at least a transponder and a radio (more than likely two radios). 

The Garmin GTX330 is a 250 Watt transponder and is fairly common in the Archers I fly. 

Your run of the mill Garmin GTR 225 radio comes in 10 and 16 watt versions.

To get the total requirement just add up all of your components draw. Most manufacturers list power requirements (or voltage and current draw requirements) for any given piece of equipment and you will have your required power. 

As for De-Ice requirements this depends on the type of system you implement. A weeping wing will only require power to drive the fluid pump and the windshield heater where as a boot style system will require enough power to drive the compressor that inflates the boots. Although its becoming increasingly more common to see FIKI on GA planes the majority of the small aircraft fleet still lacks FIKI capability.