Can I get a 375cc moderate plus Mentor implant? I'm 5'3 and 27 years old. (photos)

Thank you for the question. 
 Based on your photographs, I think that you are starting at a good place and should have a very nice outcome with breast augmentation surgery. 
I agree with you: 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 "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. 

Thank you for your question.
The way you choose an implant is as follows: First it is important to know what your goals are I.e. what cup size you would like to go to. I am ASSUMING that you want to be a C cup. The next thing one wants to do is to measure the width of your breast. This is very important. You do not want an implant that is too narrow, because you will have less cleavage and less superior pole fullness. You also do not want an implant that is too wide for your chest because it will not sit naturally and will extend too far to the side. Once your surgeon has this measurement, then one will then be able to pick out the proper profile implants. Because I have no measurements on you, I can not tell what implant profile will fit you the best. For more information please watch the enclosed video. I hope you find this information useful