Violins are available in 8 different sizes: 4/4 (also called full size), 3/4, 1/2, 1/4, 1/8, 1/10, 1/16 and 1/32. Size 4/4 is the biggest and size 1/32 is the smallest. All adults, regardless of their size, use the size 4/4 violin.
To measure what size violin best suits you, you need to know the length between your neck and the middle of your left-hand palm (when your hand is fully extended and raised perpendicular to your body, just like holding a violin). Most of the teachers prefer students to use the length from the neck to the wrist for measurement instead of the neck to mid-palm approach. The violin size determined by the neck/wrist approach would be the size that is more comfortable for students to hold. The violin size determined by the neck/mid-palm approach would be the biggest size students should use.
If you have a teacher, you should ask for your teacher's recommendation. If you don't have a teacher, we would recommend using the neck/wrist approach for students not using full size; for students who are deciding whether to used size 3/4 or size 4/4, use the neck/mid-palm approach. This is because it is always better those students feeling comfortable holding and playing the violin. However, while deciding between size 3/4 and size 4/4, if neck/mid-palm approach allows for size 4/4, then buying a size 4/4 is more economical since you don't have to buy another bigger size violin later. This is completely based on economical consideration. You should still decide what best suits your need.