Hello,
I would like to clarify a few things for you to help make your experience a good one, and not filled with unhappiness or regret.
No surgeon can guarantee a cup size for two main reasons. First, there is no standardization of bras. In other words, any given cup, like 'C' for instance, can have a slightly different shape or size depending on the bra model or manufacturer, and this will influence how a woman feels the bra fits her. Another problem is using a 'formula' to calculate the required implant volume to change a woman's cup from her current size to her desired size. Each woman's breast will accommodate a breast implant differently, some will expand and allow the implant to project to it's fullest extent, others will 'crush' the implant into a flatter profile, and this will have an effect on how a bra will fit.
So, how will you know that you have successfully communicated your aesthetic goals to your doctor? There are a number of ways, some high tech, some low tech, all helpful. I prefer to have women bring in photographs of models/celebs whose 'proportions' they like, and also some that they definitely don't like too. That gives me a good idea of your personal vision of how you would like your new breasts to appear relative to the rest of your body. This technique alone is about 95% accurate. Another technique I employ is the use of a special bra and 'sizers' that fit into the bra over your breasts. Putting this on underneath your favorite tops while looking in the mirror will let both you and me know how you want to look. Once that is established, an implant size range is chosen. Finally, there are computer controlled photographic systems that can morph your breasts into any size you want based on a given implant size that is input into the program. Really cool and wiz bang, but you are missing the full visualization of how your entire body will look in clothing. Combining any two techniques will get you as close as you can to giving the doctor an understanding of where you want to go.
OK, now the bad news. It's a bummer when reality strikes, but there is no getting around it. The problem is you are small breasted and thin, and you want to change your cup by 3-4 sizes up. It's not that it's impossible, it is possible, but people like you who want such a large change face the greatest risk of having either early or late term complications after your surgery. The two most common problems you will face are rippling and capsular contracture. Although rippling can be minimized by using silicone gel implants, it does not guarantee the prevention of rippling, and it typically gets worse with time as the implants continue to exert their force against your straining, over-stretched tissues. Capsular contracture is considered an early complication, but your risk is increased not only early but also later too.
I hope you visit a few surgeons who are qualified and very experienced too. Your discussion with them will help you make a decision about persuing your goal size or compromising to lower your risk of complications.
Best of luck!