Gains are pretty even between the woodwind instruments and string instruments families. Either will give good gains in both length and girth if employed dilligently. Cymbals, rattles, kazoos, and other overly simple instruments give little in the way of gains. Glockenspiels and xylophones, whilst technically percussion instruments, give gains comparable to the strings. This is good as percussion instruments have problems as far as PE goes.
Nobody really knows why but excessive drumming tends to actually shrink the penis. Use of a drum set is by far the worst, with few professional drummers measuring over 3”s in length.
Pipe Organs seem to be the best in the long run. Gains are very slow, almost imperceptible. But there is no change in the time invested to size increase ratio. Daily use of a Pipe Organ for a few hours will make you longer by about 5mm/year whether it is your first year or your 40th year. You can see why career organists often take up drumming later in life.