Two years ago I bought a D5100 after being switched to Nikon with a D7000. The sensor sizes are identical, the sizes of the focus points - or should I say "focus areas"? - are not.
100% view of both "focus point" marks shown in Aperture. Height of D5100 focus area is 1/7, width 1/3 of the D7000 area.
After 2 years using these cameras I still don't have a certain feeling about "what happens in these areas"? The small areas of the D5100 are sometimes great, when too much elements in different distances can be seen around the important focus target, so i can precisely aim at. And hope, I didn't move too much before pushing the shutter button. It's difficult to aim with tiny points.
On the other hand, although it's easier not to miss the focus target with the big areas of a D7x00 / D800, sometimes I really would like to know how I can help the AF doing the best it can do? What to do with targets which are smaller than the focus area? Which spot in this area is really deciding about sharp or second best sharp? I was looking in the manual and in a book about D7000, but this information is somehow hard to get.