The screen you're asking about is a little different I think compared to some of what 00rooster explained. It will ask for those grid settings regardless of if you made it via contour or grid in the map screens. Your screenshot is during export and it will ask that regardless of which style you made your map since it's exported via grids to nearly every monitor. There's a lot of opinion involved in those sizes, so I'm guessing that's some part of why there isn't much explicit help on that subject. If you called or emailed and asked, I'm sure they'll tell you. 

Also, many controllers will use the rate the actual receiver is "in" if you're splitting 2+ different rates. In other words, using the rate you're sitting in, not some far edge of your spray boom or planter, etc.