the settings you require are in the shooting menu, you will find 2 options which are relevant to your question.
Image quality - (choose between)
- NEF (RAW) + JPEG fine
- NEF (RAW) + JPEG normal
- NEF (RAW) + JPEG basic
- NEF (RAW)
- JPEG fine
- JPEG normal
- JPEG basic
and Image Size - (choose between)
- Large (4,928 x 3,264; 16.1M)
- Medium (3,696 x 2,448; 9.0M)
- Small (2,464 x 1,632; 4.0M)
you can select the file sizes you want from these options, NEF (RAW) files retain a lot more data and are therefor considerably larger files than the JPEG versions of the image.
As far as I am aware there is no way to alter the aspect ratio of width and height other than the Large, medium or small pre-sets available.
hope this helps