The size of the autofocus sensors is not adjustable but the way in which the camera uses them is. If you are close enough to the hummer to fill the autofocus icon (which does not correspond to the size of the sensor - the sensor is likely larger) then you should be able to persuade the camera to acquire focus. Keeping focus is another matter. Here, there is a distinct advantage to thumb button focusing since one can make an exposure without engaging the autofocus as a byproduct.
For the hummer it would probably be profitable to set up the camera thus:
a1 AF-C priority: release (default)
f4 Assign AE-L/AF-L: AF-On
Focus button (on lens mount): Command dial: AF-C Sub-command dial: d9, d21, d39 or 3D
With the 3D option, the camera will pick up the color of the object under the initially selected autofocus sensor and use that information to track it in the frame. A green hummer in front of an equally green background might give it some fits. With any of the dxx options the camera will also attempt to track the original object under the initially selected focus sensor. The camera will not necessarily report which sensor it is using via the viewfinder - you will have to take the tracking on faith. The camera will record the sensor it used and can be made viewable as an option in playback or in Nikon software.
The camera will now focus only with the thumb button - a shutter half-press will not do anything for you, focus-wise. If you are in the habit of handing the camera off to others, it might be a good idea to put the thumb button settings in one of the User banks (U1, U2) and maintain the camera in your normal mode when out of User mode. One advantage of thumb button focusing is that you don't have to press gingerly to avoid exposing a frame. If you are judging a hummer, this can be useful if you get it in front of a distinctly different patch of bg. The other major advantage is that, once you are satisfied that the focus is the one that will be satisfactory, you may then release the button and lock focus, thus gaining the ability to recompose at will. All this may take some learning but, for many, it is the preferred method of engaging autofocus. Me, too.
Bracketing is engaged using the BKT (lower) button on the upper left side of the lens mount , directly above the lens index stud. Command dial turns it on/off (I find it easiest to turn it off by running the command dial several clicks to the left to be certain it is at its stop and then two clicks to the right - this way I can set bracketing to zero without having to drop the camera from my eye), Sub-command dial selects for exposure differential.