The smallest gauge available for a nose stud is 20 gauge though the slightly larger 18 gauge is more common. With body jewelry, the larger the number, the smaller it is.
Most piercing professionals prefer piercing the nose with an 18-gauge or 20-gauge stud because they are small and therefore less painful. The recovery time is also shorter after using a smaller gauge.
The standard nose piercing with studs goes through the nostril. Due to the type of jewelry used for this piercing, a needle is used as opposed to a piercing gun. Once the piercing heals, it's easier to choose different sizes or styles of nose piercing jewelry.