When sizing generators, several factors come in to play.
1) Total continuous load. Add the lights, TV, fridge, etc. all together. If you don't know the wattage of appliances, figure 800 watts for the fridge, 1000 for the furnace, 200 for a TV, and lights as the bulb wattage. Figure 1000 watts per HP for the well pump. Use 1.5 times this total.
2) Starting surge: Generally speaking, figure the motor HP X 3 = the KW rating needed to start it. For example, a 2 HP motor will need a 6000 watt (6KW) generator to start it, but only 2000 watts to run it. This starting surge must be added to any continuous load expected to be in use when the pump starts. A fridge takes about 2000 watts to start.
Use the larger of these two figures. Since several things will be on at the same time, oversize the generator a bit.
When the well pump or furnace starts, expect the lights to get VERY dim for a second or so.
If you are willing to manually control larger loads (like the well pump, fridge, and furnace), you can get by with a much smaller generator.
If this is a generator-only system, simply wire the generator to the panel. If there is utility power present, a transfer switch is needed. Be very careful to install the transfer switch properly, and if possible, get it inspected by the local building department. If the generator back-feeds into the power line, and a lineman is injured or killed by a non-code installation, you'll be held completely liable. A singed-off inspection by the local building department pretty much absolves you of any responsibility if anything bad happens.