1) Large/Fine: Approx. 4.6MB (4,368 x 2,912)
RAW: Approx. 12.9MB (4,368 x 2,912)
2)ppi (pixels per inch)
3)getting more quality.
photograph using Raw mode.
After shoots:
Using good softwares, like Photoshop, Aperture and lightroom.
There are enlarging programs that will help a bit with high
quality enlarging algorithms (eg Qimage), but basically, there's a
limit.
good luck