Is 260cc Too Small for Breast Augmentation?

The size depoends on your body frame and the size that you whish to be.  The rice test is helpful.  You can look this up on line and try this test.  If it seems small to you then it will be small.  Talk with your doc.

It is very important to communicate your size goals with your surgeon.  In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as “natural” or “C cup” or "fake looking" means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on who makes the bra; therefore, discussing desired cup  size may also be inaccurate.
I use  intraoperative sizers and place the patient in the upright position to evaluate breast size. Use of these sizers also allow me to select the press implant profile (low, moderate, moderate plus, high-profile) that would most likely achieve the patient's goals. The patient's goal pictures are hanging on the wall, and allow for direct comparison.
I have found that this system is very helpful in improving the chances of achieving the patient's goals as consistently as possible.
By the way, the most common regret after this operation, is “I wish I was bigger”.
I hope this helps.

You are more petite than the average candidate but your size selection depends on several factors including breast diameter, existing breast tissue and skin compliance, etc. So there are no absolutes and certainly no right and wrong sizes. Recommnendations are made by your experienced plastic surgeon.