You need to determine what all you want to run on it, add up the wattage, then add 25 - 50% more to that and start there. You'll also need to get a transfer switch to connect into your electrical panel unless you want to run extension cords everywhere. Or You can get a "whole house" genset...
Its also just the right size to out in an old scope mount and use as a cheap tactical light! Have one on my ar currently...well over 1000 roinds and still works perfectly!
if you are doing a whole house generator, I would use a small online (NOT standby) ups on each piece of gear you are worried about. Most modern generators are cleaner on the output than in years past, and most electronics are better at handling dirty power now..