Personal preference dictates the length and diameter. If you only get one needle, get a size 6 or 7 Sharp.

Needle sizes are just the opposite of what you would expect. A size 8 is smaller than a size 6. A size 4 is long and thicker, size 12 is really thin and short. Different brands vary slightly in diameter and length and shape of the eye, but for the most part: a size 6 or 4 or 12 is about the same size no matter the brand.