Most instruments all one size fits all, however a few instruments like Violins, Violas, Cellos and Baritones do have options depending upon the child's size.  Below is a brief explanation on how to size your child for these instruments.

Violin - With arm fully extended measure from the middle of the palm to base of the neck.  Below in inches gives you your size.

1/4 Size = 17" - 18.5"

1/2 Size = 19"-21"

3/4 Size = 21.5 - 23"

4/4 Size = Anything greater then 23"

Viola - With arm fully extended measure from the middle of the palm to base of the neck.  Below in inches gives you your size.

12" Viola Size = 19" - 21"

13" Viola Size = 21.5"-23"

14" Viola Size = 23" - 24.5"

15" Viola Size = 24.5" - 25"

15.5" Viola Size = 25" - 26"

16" Viola Size = 26.5" Or larger (Note This also has to do with comfort level when you get this large)

Cello - Is a bit more difficult but below is a rough guide we will be happy to verify in person your size.

1/4 Size = 18" - 20" Generally age 5-8

1/2 Size = 20" - 22" Generally age 8-11

3/4 Size = 22" - 24" Generally age 9-13

4/4 Size = 24" and up Generally age 11-15 and up

Baritone- Comes in 2 sizes Regular and 3/4 most middle school kids will use a Full Size Baritone, often beginning students ages 8-11 will use a 3/4 baritone.