There are more factors to keep in mind than just the 'size' of the breast cosmetically. You will need to keep in mind the look of the breast (natural or fake), your lifestyle (how active are you?) and the risks associated with the size of the implant.
Some women will want to go for the 'fake' looking breasts - you know the breasts that are very round and don't have a natural slope to them. Others will want to go for the natural look, just larger. Others will want to go for something in between. This is something you will need to discuss carefully with your surgeon to make sure you are happy with the end result.
Regarding the size itself, you probably have read that sometimes breast implants can interfere with sports like swimming, yoga, tennis, golf etc. The bigger the implant you go for, the more likely you'll have problems with the breasts 'getting in the way'.
Lastly, the larger the implant you get, the larger the risk of complications and detrimental changes: e.g. breast sagging, palpability, less natural appearance, numbness, arms rubbing against side of breasts, ache, and future malposition.
As Roland mentioned, your surgeon can talk you through the different options as well as show you computer images; but in addition to this I'd suggest you ask them for a trial implant to take home and use under your clothes for a day or two to get used to what the final product will look like and how it works in your life.
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