I am 5'8 and 140 pounds and I am going in to have breast implants. I have chosen 550 cc. Will they be too big?

Unfortunately,
you have not told us your bra size just your cup size.If you are 32 or 34 each 100 cc’s of implant
corresponds to 1 cup size change.If you
are 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.500 or 550 cc implants are large, will not fit retro-pectoral, will
extrude inferiorly and laterally requiring revision.22% of women who undergo breast augmentation
alone require revision within 2 years because they are dissatisfied with the
result.A frequent cause of
dissatisfaction is over implantation.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.Implant profile is irrelevant in the
retro-pectoral position since the muscle compresses it.Since you are 21, you cannot have silicone
implants, only saline implants.Best Wishes,Gary Horndeski, M.D.

550 cc breast implants are very large.  Generally speaking having breast implants larger than the 450 cc range can increase your chances of complications.In my experience in average-sized women 550 cc implants look large and often obvious as implants.Please reconsult her plastic surgeon for measurements of your breast and chest which the surgeon can use to evaluate best size considerations based on a table given by the implant manufacturers.  I also suggest that you visit the manufacturers website such as loveyourlook.com to see photographs of patients who have had implants that large.

Thank you for the question. 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 for my frame” 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.