I am considering 500 cc saline implants but wondering if it's too big? (Photo)

Your implant choice is a very personal one, and what may be too small or big to you can be very different for someone else. It is difficult to tell based only on a front view of your photo how much fullness you need. Typically 500cc implants will result in the patient wearing a D or DD bra size (or even larger if she started with more breast tissue) depending on the bra manufacturer since there is no standardization of bra cup sizes. Also, larger implants will make you appear a little heavier so you may want to take that into consideration.

Thank you for the question and picture. Without side view pictures, I can not tell whether or not you may benefit from breast lifting surgery as well. You would be wise to achieve a long term stable weight before 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'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 "C or D cup” or "too big" 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 technology may also be very 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.