How much smaller than my BWD can my implants be? I have a Wide BWD but I’m intimidated by heavy cc’s. I don’t want sag!

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. I agree with your thought process. My very general advice when I first meet patient seeking breast augmentation surgery:  aim for ONLY as large as necessary to achieve  an outcome they will be pleased with long-term ( "bigger is not necessarily better").
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 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.

Thanks for your question. It is true that your BWD or Base/Breast Width Diameter is very important. Your BWD essentially determines the size of implant that your breast can accomodate before the implant starts to become more noticeable beyond the natural shape of your breast. It is hard to say without seeing you in person but it would probably be worth you asking to have a proper sizing appointment where you can try on different shapes and sizes of implants which will not only allow you to see how they look but you can also feel how heavy they are.You can calculate yourself averagely how heavy different implants would weigh; 
A quick way is to take the weight of both implants and then divide by 426.67. Example:
300cc x 2 = 600cc 600cc ÷ 426.67 = 1.406 lbs.