Breast implant size 210 or 240 too small? Flat chested 26 y/o. (photo)

I have some very petite patients with a figure very similar to yours. We have done implants ranging from 210 to the upper 200 cc's. All have been quite pleased. The size depends on your breast width and whether or not you want a full appearing upper breast or a more natural slope to the breast. There are high profile and ultrahigh profile implants that will allow you to choose more volume without being too wide for your breast. I would suggest trying some of these on with a sports bra before making a final decision on size. Good luck!

In my practice, I use Crisalix 3D simulation with which you can see what size of
implant would suit you and how you would look post the surgery. So depending
upon whether you choose 200 cc, 220 cc or whatever size you choose, you can see
how it would look on your body. I would recommend you go for 3D simulation;
patients really appreciate that.

Patients often think in terms of cup size when considering augmentation. Unfortunately, devices are sized in terms of milliliters (cc) of volume. This can lead to some confusion when sizing. Additionally, it is important to remember that cup size itself is not standardized with variations from one manufacturer to another. Unfortunately, as many women can attest their cup size in an industry leader such as VS is not necessarily transferable to another brand.
Another point which is often under appreciated is that of anatomy and starting point. Any implant will add volume to the volume which is already present. The implant is additive. A particular volume will not necessarily confer the same cup size to different patients (often times it will not even confer the same cup size to different breasts in the same individual...remember they are "sisters" not "twins"). 
A general rule of thumb is that 125cc can represent somewhere between 1/2 to a full cup size increase. Smaller volume differentials (25-50cc) are typically less consequential representing a volume change of less than a shot glass. However, I have found these numbers, at least anecdotally, to be of little help. Patients often present with notions/goals which do not correlate with these sorts of sterile volumetric assessments.
When sizing patients, there are a number of useful tools including:
-3D imaging (has the added benefit of offering a volumetric analysis of the pre-operative breast)
-Breast sizers (rice bags)
-Goal photos
I also recommend that patients commit to a particular look rather than a cup size. Once a patient settles on a look that pleases them the overall cup size increase becomes less relevant.The key to obtaining a natural result is to stay within the parameters defined by your BWD. This will ensure that you avoid the dreaded "fake" look. 
With regards to your particular choices, 210 to 240 cc sounds reasonable. While there are no cup size guarantees, if you are happy with the look, you are unlikely to be disappointed post op based upon the cup size.
As always, discuss your concerns with a board certified plastic surgeon (ABPS)

Dear LouH26,Breast implant type and sizing selection is a complicated process and requires a detailed consultation that includes a discussion of your goals, a detailed examination, and a lengthy discussion. But the decision will always start and end with the patient. When it comes to sizing, we have all of our patients find 3 photos of their desired breast size. We ask for naked breast photos because clothing can obscure the true size (push up bras). Next, we consult with patients and go over the photos and take detailed breast measurements during a physical exam. Next, we using a sizing algorithm to determine a range of implants that will work based on the exam and the measurements.Lastly, during the operation, we use silicone sizers in the operating room to see how the size actually looks in the breast. We sit the patient up in the operating room, while asleep of course, and compare the sizer result with their desired look photos. If it is too small, we start the process over with a slightly larger implant. We don't stop until we get the perfect match.Of all of my breast augmentation patients, every one of them has been happy with their size due to this detailed and meticulous process. You don't want to invest time and money into a result that is either too big or too small.You should discuss your concerns with a board-certified plastic surgeon and make sure that your surgeon understands your goals.Daniel Barrett, MD, MHA, MS 
Certified, American Board of Plastic Surgery  
Member, Am. Society of Plastic Surgery

Hello and thank you for your question. You are a great candidate
for a breast augmentation.  240cc or slightly larger will likely accomplish your goals.   The size, profile, and shape of the
implant is based on your desired breast size/shape, your chest wall
measurements, and soft tissue quality.  This decision should
be based on a detailed discussion with equal input from both you
and your surgeon.  This entire surgery
can be performed with a small incision technique.  Make sure you
specifically look at before and after pictures of real patients who have
had this surgery performed by your surgeon and evaluate their results.   The most important aspect is to find a
surgeon you are comfortable with. I recommend that you seek consultation with a
qualified board-certified plastic surgeon who can evaluate you in person.
Best wishes and good luck.
Richard G. Reish, M.D. FACS
Harvard-trained plastic surgeon

Thank you for your question and posting appropriate pictures. You seem to be a great candidate for breast augmentation. The 260cc implants will likely make you a full B cup, You would likely need 300+ cc to be a C cup. If you’re choosing to go with smaller implants, I would recommend low profile to maximize cleavage. Best to have a consult with Vectra 3-D imaging to simulate the postoperative outcome.

Based on your photos and measurements.  Those implants are on the small size to achieve your goals it seems to me but they are close and without an in person evaluation it is not possible to definitive statement.  Using the smallest implant that will achieve your goals will give you the longest lasting results with the lowest chance of complications.  Make sure you choose a board certified plastic surgeon.  Good luck.