Inside tag says 12 for size. I found a photo on a similar one online.
Thank you for submitting your item for appraisal. This appears to be exactly what you suggested. The additional reproduction of the late 1960s advertising you found really brings to life the kodak coloured - summer of love -beach boys - feeling of that era. Vintage swim suits in general do not sell as well as evening dresses, even if they are so iconic as the one in your possession. The estimate is based on similar sold at auction. A serious collector might be willing to pay more than the high estimate.