The violin is the highest sounding instrument in the string instrument family.  It is also the smallest instrument and is held under the chin with the aid of a shoulder rest. 

The viola is played exactly like the violin. It is slightly larger and sounds lower than the violin

The cello is a much larger and lower sounding instrument in the string family. It is so big that students have to sit down to play. Keep in mind this instrument has to go home every week, so think about carrying it back and forth to school before choosing this instrument. 

The bass is the largest of the string  instrument family. It is so large that a student has to stand when playing. If you walk or ride the buss to and from school this would NOT be the best choice.