How large can whiteheads grow?

There is no defined size limit that a pimple can be. However, most standard white head pimples (also known as closed comedones if you want to look them up), do not get larger and larger without being infected. It sounds like yours needs checked out before it becomes a full blown abscess. Whiteheads occur when there is a blockage fo pores by dead skin cells and keratin. Sebum (a natural skin oil) then builds up in the pore causing it to enlarge. the sebum production is largely governed by androgen hormones which is why acne begins to get worse near puberty. If the sebum stays isolated, it remains white (hence the name white head). Finally, sebum is a great place for bacteria to grow making the pimple larger and more inflamed. At some point, the pimple will be considered a follicular infection or abscess and may require incision and drainage. If this pimple (or follicular infection) is still bothering you, it may need to be lanced by a physician. I suggest you schedule an appointment with you primary care physician anyway to discuss the management of your acne and to make sure that you are currently being given optimal treatment. Good luck.