300cc too small? I am almost 5’9 and a size 2/4 with a 34a bra size.

Depending on the individual anatomy and desired results, estimations, measurements and surgical planning should be made for each patient individually. It will be best to consult a board certified plastic surgeon who is experienced in cosmetic breast surgery in person and discuss your goals and options after a physical examination.Hope this helps,Ali Rıza Öreroğlu, MD FEBOPRASEuropean and Turkish Board Certified Aesthetic, Plastic and Reconstructive SurgeonIstanbul, Turkey

Thank you for your question! I would make sure you do a couple "sizings/fittings" to decide what size looks good on you!  Make sure you consult with a board certified plastic surgeon and discuss your desires and results! Best of luck!Dr Dhaval PatelDouble Board CertifiedPlastic SurgeonChicagoHoffman EstatesOakbrookOrland Park

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. 
Regarding your specific question, it is nearly impossible to predict cup size. For a smaller framed individual, I will typically recommend a high profile device (greater volume for a given BWD).
As always, discuss your concerns with a board certified plastic surgeon (ABPS).