Every fall, many parents of fourth graders rent an instrument for their child’s school music program. The task is particularly daunting for those renting a string instrument.   Although introductory band instruments come in one size, string instruments come in many sizes, with violins ranging from 1/32 size to 4/4 (or full) size. Most fourth graders need a ½ or ¾ size. To make violin selection more confusing, a ½ size isn’t half the size of a full-size instrument. Rather, for violins, a…