For a natural look on a petite frame, what size would you recommend: 250/275cc or 275/300cc Mentor HP?

Thank you for the question.
My best advice: more communication.
It is definitely worth spending more time (prior to the date of surgery) with your plastic surgeon to discuss further.  As you know, it will be important for many reasons, that you feel comfortable that you and your plastic surgeon are on the "same page" prior to proceeding.
 Ultimately, careful communication of your goals (in my practice I prefer the use of goal pictures, direct examination/communication in front of a full-length mirror, in bra sizers, and computer imaging) as well as careful measurements (dimensional planning) will be critical.
Generally speaking, the best online advice I can give to ladies who are considering breast augmentation surgery ( regarding breast implant size/profile selection) is:
1. Concentrate on choosing your plastic surgeon carefully. Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. Ask to see lots of examples of his/her work.
2. Have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will be critical in determining breast implant size/type/profile will most likely help achieve your goals. 
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. For example, I have found that the use of words such as “natural or fake look” or "C or D cup” etc means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on him who makes the bra; therefore, discussing desired cup size may also be inaccurate. Again, the use of computer imaging has been very helpful during the communication process, in our practice.
3. Once you feel you have communicated your goals clearly, allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals. Again, in my practice, this decision is usually made during surgery, after the use of temporary intraoperative sizers.
I hope this helps. Best wishes for an outcome that you will be very pleased with. 
 

After I measure and discuss goals with my patients, we then look at pictures of women with similar measurements with different size and style breast implants. My patients tell me that this process is very helpful in determining what size breast implant is right for them. 
Dr. Ted Eisenberg, Board Certified Plastic Surgeon and Author                     
RealSelf Distinguished Hall of Fame Inductee

You have a choice, either natural look or more size. You don't get both. If increasing breast size is your priority, then choose high profile. If you really want a breast with a soft upper pole and more moderate projection, then choose an intermediate profile. Also, the bigger the implant, the faker the look.