What is the right size brad nail for crown moulding?
In most cases you can use 18-gauge 2-in brad nails. When choosing the correct size nail you want to consider the thickness of the crown moulding, the thickness of the stud and  the thickness of your drywall.  In most cases your crown moulding will be 1/2 inch with 1/2 inch drywall. At least half of your nail should be in the stud.  If your crown moulding or drywall is thicker than 1/2 inch you should consider a longer nail.