you really need to look at your sump pump and figure out its wattage (normally stamped into a nameplate on the side of the electric motor).
Or, you can usually translate pump HP to wattage (not directly though - motors are not 100% efficient. a 1hp 120v motor needs 16A = 120v * 16A = 1920w or 2kw
http://www.elec-toolbox.com/usefulinfo/flamtrcharts.htm
Add 100w per light bulb, and you should be good -
I'm guessing you'll need a 3 or 5kw generator. a 10K generator is what most people buy to power thier whole house (incl A/C)
-John