As in the case of brass instruments, it is often wise to encourage students to take up the instrument that best fits their physical characteristics.  Small and short students would probably be better advised to avoid the bassoon or baritone saxophone, for example.  Clarinet players need to have fingers large enough to cover the tone holes.  Saxophone players do not need to cover tone holes, but they do need hands that are large enough to reach around the larger instrument. 

It is also important to observe the general shape of the teeth.  Crooked teeth can cause the mouthpiece to shift off-centre.  An excessive overbite can also create difficulties, although such problems are probably not as serious as those facing brass players.  The flutist with an excessive overbite may not be able to form a sufficiently small aperture, while the clarinetist with similar physical characteristics may not be able to produce a good tension seal around the mouthpiece. 

It is important not to exaggerate the importance of physiology, however. In most cases, interest should be the most important factor.  Orthodontic treatment may sometimes be required, but most students with a reasonable degree of enthusiasm for their chosen instrument can usually adapt well enough.