Opinions: implant diameter 0.5cm-1cm under or over BWD?

Generally speaking , most surgeons will recommend an implant no greater than the base width of the breast. Even a larger implant placed correctly shouldnt cause symmastia. This problem occurs most frequently when large implants are placed over the muscle and the surgical dissection is carried to far towards the midline. It can occur in submuscular placement as well if the muscle is excessively dissrupted. Over sized implants can contribute to a lot of problems, and I dont recommend using them.

hello thank you for your question and provided information as well.. based in your question is difficult evaluate you if the case dont have a picture ; my recommendation is get a consultation in person with your plastic surgeon to discuss all about the theme.

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. It is important to remember that smaller individuals rarely have exceedingly large breasts, thus for those seeking "natural," conservative sizing is best.
With regards to your specific question, there are no hard and fast rules. However, exceeding your BWD typically yields a less "natural" more "augmented" look. Typically, in the setting of a submuscular augmentation, excess width ends up lateral in the armpit. Ultimately, the decision is one which is based upon expectations.
As always, discuss your concerns with a board certified plastic surgeon (ABPS).

The concept of biodimensional breast implant planning takes into account the overall width of the breast coupled with the thickness of the breast tissue. Implants should be sized such that they fit inside this footprint. In other words, choosing a base width that is ~1cm less than the breast width. Choosing a larger implant does not mean symmastia will occur, that happens from overdissection of the medial pocket. If the implant is larger than the breast width, two things typically happen - 1. The implant pocket is not made wide enough which causes chronic folds of the implant and can lead to a higher rupture rate2. The implant pocket is made to the implant size and the breast comes off the outer side of the chest.Hope that helps.

Thank you for the good question.  There are no hard and fast rules when it comes to diameter of breast implant versus diameter of breast (band width). In other words, these measurements are useful but are only one of the many factors come into play when it comes to selection of appropriate breast implant size/profile.
Having said the above, you are correct: every patient undergoing breast augmentation surgery should understand that their anatomic starting point will limit the outcome of breast augmentation surgery. The use of a larger breast implant (than what is safely "allowed" by the patient's anatomy) can be problematic;  potential problems include breast implant displacement/malpostion issues ( such as bottoming out, lateral displacement, symmastia...)  and/or significant breast implant rippling/palpability.
Other factors involved include surgeon dependent factors: for example, careful breast implant pocket dissection and careful selection of breast implant size/profile.  Remember (when it comes to breast implant pocket dissection and achievement of ideal breast implant cleavage) that each nipple/areola complex must be centered on each breast mound.
Ultimately, careful selection of plastic surgeon and 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 know 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.