How do I determine what size Generator I need?
You'll have to decide what you want to run (Refrigerator, furnace, TV, air conditioning, etc.). Then you'll have to determine how much power each item requires. 

To determine how much power each item uses, you can use an estimate like this one provided by Energy.gov


  TYPICAL WATTAGES OF VARIOUS APPLIANCES
  
  
  Aquarium = 50–1210 Watts
  Clock radio = 10
  Coffee maker = 900–1200
  Clothes washer = 350–500
  Clothes dryer = 1800–5000
  Dishwasher = 1200–2400 (using the drying feature greatly increases energy consumption)
  Dehumidifier = 785
  Electric blanket (Single/Double) = 60 / 100
  Fans
  Ceiling = 65–175
  Window = 55–250
  Furnace = 750
  Whole house = 240–750
  Hair dryer = 1200–1875
  Heater (portable) = 750–1500
  Clothes iron = 1000–1800
  Microwave oven = 750–1100
  Personal computer
  CPU - awake / asleep = 120 / 30 or less
  Monitor - awake / asleep = 150 / 30 or less
  Laptop = 50
  Radio (stereo) = 70–400
  Refrigerator (frost-free, 16 cubic feet) = 725
  Televisions (color)
  
  19" = 65–110
  27" = 113
  36" = 133
  53" - 61" Projection = 170
  Flat screen = 120
  
  Toaster = 800–1400
  Toaster oven = 1225
  VCR/DVD = 17–21 / 20–25
  Vacuum cleaner = 1000–1440
  Water heater (40 gallon) = 4500–5500
  Water pump (deep well) = 250–1100
  Water bed (with heater, no cover) = 120–380 
  


You could also check the nameplate of each device, which should either list wattage directly or provide enough information for you to calculate it. If you have to calculate wattage from the nameplate, Ohm's law will be your friend.

Watts (P) = Volts (V or E) x current (A or I)