Using two different (silicone) implant sizes for slightly different boobs?

If it is not noticeable now and does not bother you, then using the same size will probably be the same, trust your surgeon's advice. Good luck.

Thank you for the question.  I find that I use different sizes of breast implants for an individual patient undergoing breast augmentation surgery quite frequently.  Sometimes, when there is significant chest wall asymmetry as well, different profiles of breast implants may also be helpful. Of course, if there is very minor breast degree of asymmetry, use of different sizes of breast implants is not indicated.  My patients will also hear me say: "sisters not twins", despite best efforts.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.  This additional communication will go a  long way when it comes to helping alleviate your anxiety 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” 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.

Thanks for your inquiry, and this is a common situation.  In general terms, most implants come in sizes 20-30 cc apart in implants under 500 cc, over 500 cc the implants come sometimes in 40 plus cc increments.  If the difference between you breast is closer to a size difference in the implants then pick different implants, hope that helps and best of luck.