It's not neccessary to have a butt like Kim K but if you have a nice butt then it's a bonus. But again if a guy doesn't like you just because you have a smaller butt then he's not even worth your time.
Uhh... wow... Umm, if you google "great butt" and look at the images, that's show you lots of examples of "nice butt" and very quickly you can sort of get a feel for what guys conciser to be a nice butt on a girl. Especially if you turn off the safe search. There shouldn't be too many... things you maybe don't want to see, and it will provide a better sample.