Ear posts are actually secured by the curved side wings of the nut, not by the hole itself.
To choose the right size of ear nut for your post, look for one that is complementary in weight to the earring it will support. If the earring is large, select a nut with a front surface that has a larger diameter to help support and steady the earring when it is worn.
Select a nut with a hole size that is slightly larger than the post diameter. The nut should be as close in size to the post as possible while still being slightly larger. Threaded ear nuts should closely match the diameter of the threaded ear post they will be used with.