Violins are available in 8 different sizes: 4/4 ( 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 size, use the full size 4/4 violin.
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. The reason is it's always better to feel comfortable holding and playing the violin. However, in 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 could save you money since you don't have to buy another bigger size violin later. This is completely for practical reasons. You should still decide what best suits your need.