I'm 5'3 and 110 lbs. I'm suggesting 300cc mod plus silicone implant. Am I choosing the right size? (photo)

We normally have the patient size and select her implant volume, provided that it doesn't create a hazard for her- that is in your case I would not insert anything over 450CC. We have the patient sign a consent for surgery that includes the implant information. 25CC is hardly a significant volume difference. Also, it is more common that a patient wishes that they had chosen a larger implant than wished to have gone smaller. With your height and weight- I would recommend HPThe final volume should be yours in writing.

Thank you for the question and picture. Based on your photograph, you are starting at a great place and should have a very nice outcome with breast augmentation surgery.   In regards to more specific advice, despite good intentions, online consultants will not be able to  very helpful.  Careful measurements, dimensional planning, and careful communication will be key.
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.  Again, have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will be critical in determining which operation and/or 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. I have found that the use of words such as “natural” 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 who makes the bra; therefore, discussing desired cup  size may also be inaccurate.   Therefore, I ask that my patients not base their communication preoperatively OR  their satisfaction with the outcome of surgery postoperatively, on achieving a specific cup size. The use of computer imaging may be very helpful during this communication phase.
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.  The use of temporary intraoperative sizers, viewing the patient's breasts in the upright and supine position, are very helpful during the breast implant selection process.
I hope this, and the attached link, helps.  Best wishes.