Boothand

The value of a music note becomes exponentially smaller with the amount of stems/branches it has. So a note with one branch = 1/4, two branches = 1/8, three branches = 1/16. The duration of a 1/8 note is shorter than that of a 1/4 note, which can be taken figuratively as the note being 'faster'. The shorter the duration of the tone (i.e. the 'faster' the note), the smaller its value in musical notation (1/4, 1/8..) and the more stems/branches it has.

The confusing part is that the three properties here, duration, size and shape, all fit different aspects of the 'note', namely the sound (how fast/quick/short it is), the numerical value that the note-sign stands for (1/4, 1/8) and the shape of the sign itself. So the properties refer to three completely different things that are related to the sign: the expression (sound), the symbolic representation (value) and the sign itself (shape).

Although it works as a riddle, it is based on an equivocation of the various things that the word 'note' stands for, since a pitch doesn't have size or shape, a note on the paper does not have velocity and doesn't change its size depending on its value, and the numerical value of a note has neither velocity nor shape either.