There are obviously no absolute answers to what you're talking about. However, in my experience, I would say the following:
1) Very few patients think their implants are too large; many patients wish they had gone a little larger
2) Most women who have a lot of sagging are not as large (cup-wise) as they say they are; instead, they need a larger bra cup to hold the excess and loose skin. If you look at the actual breast tissue size (ie, imagine you've already removed some of the skin - sorry if the visual is rough) the breasts are smaller than the cup size bra they're wearing.
3) It's about 150-200 cc (or 5-7 ounces) per bra cup size. So if you're a full B and want to be a full C, you need about 250 or so. But if you're really a smallish B and want to be a very full C, you probably need more like a 300 or 350.
4) Certain types of breast lifts limit the size implant you can put in. Depending on the size of your areolas and exactly where they're positioned (nothing personal, of course) there is a limit as to how large you can go if you do an anchor or short-scar lift (not much difference between the two, sometimes).
Conclusions? No simple answers, but I'd probably go with an anchor or short scar, go a little larger than they're saying IF the implants will fit and otherwise just be happy with your result. You can always have larger implants put in at some point in the future; the key is to get a good shape and have safe healing and recovery at this procedure.
I hope that this helps and good luck,