I am going from a small B cup to 500 cc silicone implants, but I am having second thoughts, will they be too big now?

Unfortunately, you have not told us your bra size just your
cup size. If you are a 32 or 34 each 100
cc’s of implant corresponds to 1 cup size change. If you are a 36 or 38 each 200 cc’s of
implant corresponds to 1 cup size change.
From this, you can compute the volume required to achieve your desired
goal. However, 500 cc implants are quite
large, will not fit retro-pectoral, will extrude inferiorly and laterally
requiring revision. I always recommend
small round textured silicone gel implants placed retro-pectoral since they
look and feel more natural, are more stable, less likely to ripple or have
complications needing revision. In the
retro-pectoral position, the profile of the implant is irrelevant since the
muscle compresses it.Best Wishes,Gary Horndeski, M.D.

The ideal breast size is a matter of opinion and taste.  As long as you have a good understanding of what 500 cc will look like on you based on trial implants, this very well may be a good size for your tastes. In my opinion, based on your body type, 500 cc will make you look out of balance and detract from the beauty of your shape.  I would assume you would be in at least a DD bra with that implant size.  I'm not sure how larger implants would help with the distance between the breasts and in my opinion would only make matters worse.Did you use 3D imaging to help you "see" the shape and size on your body?  If not, I would recommend it.  It will either help you feel comfortable with 500 cc implants or determine a size that you like better.   Breast shape is something that is difficult to tell with trial sizers alone.

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're 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. I have found that the use of words such as   “natural” or "too big"  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. The use of computer imaging may be helpful during this communication process.
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.
I hope this (and the attached link) help. 

Pretty posted photo but not helpful, need frontal un bread posted photo. In general terms the 450 to 500 cc HP implants could be the choice if you desire D++ implants appearance.///

There are many factors and anatomic measurements beyond weight and height that should be utilized in selecting the ideal implant size and shape for any particular individual. Following the advice from a surgeon on this or any other website who proposes to tell you what to do based on two dimensional photos without examining you, physically feeling the tissue, assessing your desired outcome, taking a full medical history, and discussing the pros and cons of each operative procedure may not be in your best interest. I would suggest you find a plastic surgeon certified by the American Board of Plastic Surgery and ideally a member of the American Society for Aesthetic Plastic Surgery (ASAPS) that you trust and are comfortable with. You should discuss your concerns with that surgeon in person.  Robert Singer, MD  FACS La Jolla, California