About how big is a 600/650 cc implants? Do they vary by manufacturer?

The dimensions of a manufacturer's implants are readily available on their websites and catalogs. There will likely be some variation from manufacturer to manufacturer. However, they are typically close.As one goes up in profile the footprint or BWD of the device will shrink and the projection will increase for a given volume. I have found that full profile Allergan implants correlate closely with high profile Mentor implants. These devices exhibit higher projection than similarly sized moderate (+)/moderate profile devices. With regards to your question, a 600/650cc implant is a large implant by any manufacturers standard. The measurements/dimensions will vary most with profile (rather than brand).As always, discuss your concerns with a board certified plastic surgeon (ABPS).

Most patients think in terms of cup sizes; where breast implants are measured in volume.  The average size breast implant in the US is 350 cc; the smallest is about 100 cc; the largest 800 cc.   Available implant sizes with associated shape, volume, dimensions, projection specifications, and warranty are listed on Allergan's and Mentor's websites. The differences are subtle between brands.Kenneth Hughes, MD, Board Certified Plastic SurgeonLos Angeles, CA

 Although the volumes will not change, dimensions (width and height) of specific volume breast implant volumes will vary from one manufacturer to another and from one model type to another. For example, the high-profile implant has a greater projection and a smaller diameter compared to the moderate profile implant. The moderate plus profile implant as projection and diameter characteristics that fall between the moderate profile and high profile implants.
I prefer to have a wide selection of implants ( all profiles) available in the operating room. I use intraoperative sizers and place the patient in the upright position to evaluate breast size. Use of these sizers also allow me to select the breast implant profile (low, moderate, moderate plus, high-profile) that would most likely achieve the patient's goals. The patient's goal pictures are hanging on the wall, and allow for direct comparison.
I have found that this system is very helpful in improving the chances of achieving the patient's goals as consistently as possible. 
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” 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.