Will 250-275cc Silicone Breast Implant Result in a C Cup?

Smilee,
If you feel like you believe these blogs more than in your plastic surgeon, you don't need another implant...you need another plastic surgeon.
Almost everybody your size wants a C cup. Well, almost everybody - some unique cases come to mind but it's good to hear that you want a reasonable result. We've all done patients your size, lots. This is a very tasteful choice and your plastic surgeon probably wants you to look classy so don't get too nervous by our blog comments.. we're just blogging and you need to trust a plastic surgeon that examined you. The implant sounds a bit small to me also, but again, I haven't examined you so he knows best. Another possibility is that you may have since gained a bit of confidence in the idea and now you're comfortable with a larger implant... that happens often.
You're an A and you want to be a C. Simple right? Not so. You should visit with your plastic surgeon while there's still time to resize. Sometimes i'll get patients to try on sizers in the office to get a feel for the implants she is choosing. She'll look through the photo album and find someone with her features and we'll know what implant that patient used. Lastly, a good communication tool is to bring in photos of girls with your figure and your desired breast size. This is just an estimate, you are not using it as a guarantee, but if you show your plastic surgeon a photo of a girl with what he sees as a D and you see as just right, then he'll be using a different implant.
Think of the comments that you made during your first consultation. Those are very important to me when I'm listening to my patients expectations. If you said "I really want them to look natural on my body" or "I just don't want them to be noticeable by everyone" or similar comments, then he's trying to protect you from this social circumstance while still filling your soft tissue envelope according to his physical examination (when he measured you). It is very common to be hesitant on the size you're picking and impossible for someone on this blog to tell you what size you should be using.
It's nice to hear that you've selected silicone, I think that's a good bet since your breasts will be more implant material than breast tissue so you want it to feel as natural as possible.
Best wishes on your augmentation, you're probably going to look great with whichever implant you choose. In general, patients with your build are the easiest augmentations to get a result that looks smashing.

There is no accurate/reliable/predictable correlation between size/profile breast implant utilized and cup size achieved. For example, I do not think that the generalizations of "200 cc equates to 1 cup" hold any validity or are helpful in any specific or practical way.  In my practice, I ask patients not to communicate their goals, or evaluate the outcome of the procedure performed, based on cup size references.

Ultimately, careful selection of plastic surgeon and  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.In your case, 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.

  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  "C 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.

Final breast size after breast augmentation is very difficulty to determine as each implant will create a different size based on the patient. For example, 250cc on you may create a different bra size than a 250cc on another patient. At consultation I do thorough chest measurements on my patients as well as have them try on implant sizers and perform 3D imaging to help determine their desired implant size. If you are having second thoughts, I recommend scheduling a second in-person consultation with your surgeon to discuss your concerns. Best of luck!