WHAT SIZE MOUTH PIECE IS GOOD TO USE

Too many variables there. What type Horn? What level horn player? Thick lips or thin lips? Braces? Playing above the treble clef, on the treble clef, below the treble clef, on the bass clef, or below the bass clef? What instruments are you playing with; trumpets, woodwinds, orchestra, band?

For new and intermediate horn players? I suggest a King H-2, or Farkas MDC. They're very good all around basic work horses that have been proven again and again.