It really is a bit how long is a piece of string... with gennys the bigger the better, but it is down to your space, noise limitation and budget! anyting above 1kw will be ok, but 2kw to 3kw would be better. work out the max number of things likely to be going at once and then work out the draw. a normal kettle for instance can pull 2-3k in a spike.
Things will still work with less power, but take a long time..I know, when I had to wait 20 mins for a 4 min microwave dinner to do