Many get confused with the shape and size. One should not seriously worry about it. Just see that the Mukhi is well defined, corns and contours are natural, there are no cracks near the central hole and the bead has a good health . Smaller size is also considered to be very auspicious.
Many astrologers have also worked on Rudraksha. They suggest 'Mukhi' based on Birth Rashi and Nakshatra.However one could wear according to the needs / desires to be fulfilled
Some astrologers suggest to wear Rudraksha according to ruling planet as under:-
Sun - One Mukhi or Twelve Mukhi
Moon - Two Mukhi
Mars - Three Mukhi
Mercury - Four Mukhi
Jupiter - Five Mukhi
Venus - Six Mukhi or Thirteen mukhi
Saturn - Seven Mukhi or Fourteen Mukhi
Rahu - Eight Mukhi
Ketu - Nine Mukhi
Note : Ten and Eleven Mukhis are not under any planetary effect.