I just got my nipples pierced and..?

My piercer says when I change my jewelry to get 9/16" (14mm) but many online stores don't sell 14mm. They either have 1/2" (13mm) or 5/8" (16mm). Will I be able to wear a 1/2" or do I have to get the bigger one? I want the bar to be pretty snug, but will the 13mm barbell be too short?