Strings
Violin: sized 1/16 through 4/4. The teacher measures arm length and selects.
Cello and bass: same fractional sizing. A 1/2 size cello is for ages 8-10.
Brass
Trumpet, trombone: most kids 10+ can play full size.
Tuba: a junior-size sousaphone or compact tuba for middle school.
Woodwinds
Clarinet, flute: full size from age 9-10. Some flutes have a curved head joint for shorter arms.
Reeds and accessories
Sized by strength (2.0, 2.5, 3.0). The teacher will say which.