If you can comfortably insert any size tampon, you don't need much more space to allow for intercourse. I use two fingertips as a rule of thumb; that is, if I can insert index and middle fingers during an exam, I can assume intercourse is likely to be comfortable. The only time I've done surgery to enlarge the vaginal opening was when only a Q-Tip could be inserted--a definitive intact hymen.
What you might find helpful is vaginal dilators to help to extend the elasticity that you already have. The graduated sizes of dilators, regularly used, can gently stretch the tissue to assure comfortable penetration. I'd certainly try that before opting for surgery!
Dr. Barb DePree, M.D., has been a gynecologist and women’s health provider for almost 30 years and a menopause care specialist for the past ten. Read more about and from her here.