While skin tags are usually small (around 2mm-5mm) but they can sometimes grow up to half an inch. The growth can be natural or can be caused due to irritation from clothing or jewellery. If you see sudden growth in your skin tag, you should definitely get it checked by your dermatologists. Because it could be a different skin growth and not a skin tag that you thought it was. Dermatologists are qualified enough to either examine the growth visually. They can also take a sample for biopsy, in case there's any doubt.