I'm not sure what you mean by 'sump area'. Do you mean the area inside the catch basin itself? If that is what you are talking about, it is not important. I think the size of the grate is directly related to how much water you are trying to move, but if it all go's into a 4" pipe, then that really doesn't matter. Keeping the grate (no matter what size it is) free from debris during a rain event is more important and will mean a lot more water get's moved quickly. If you can post picture's of the problem area, maybe i can give you a more definitive answer.
This link might help - http://www.ndspro.com/