One Size Fits All for Wind Instruments!
That being said, we start children on the standard "C" flute, the Bb Clarinet, the Alto Saxophone, the Tenor Trombone, and the Bb Trumpet. There is only one size for each of these instruments. When you refer to the flute, clarinet, trombone and trumpet, this implies the above types. Saxophone has other types that are common, so the name "Alto" must be referred to.
For Bowed String Instruments, there are different sizes:
The nature of the stringed instrument's acoustical properties lend itself to smaller and larger sizes. This is good for children so the instrument can be more manageable.