M.C.
I went up about three cup sizes when I was nursing. My breasts were typically about the same size throughout, but when I went too long between feelings and got engorged they went up at least a size or two... And were rock hard. Lol. After nursing, they would seem a bit deflated for about 15-20 minutes, then would puff back up.
As far as sagging goes... I don't know many people who chose to breast feed. Out of everyone I know who formula-fed their kids, the sagginess is completely random... Which makes it hard to blame it on breast feeding when none of them did. Most went to a slightly saggier version of their old size... But nothing a decent bra doesn't hide. One of my friends, somehow, managed to gain and KEEP 3 cup sizes, with fabulous perkiness. (Bi*ch. I say that with all the love in the world.) my own experience was to gain a cup size, but they are definitely a bit saggier. But hey! I'm up a size, so with a push-up I actually have a figure now! Woo!
I will say to get a nursing bra that is comfortable enough to sleep in. I had leaking issues, and wore a bra 24/7 while I was nursing. Even during sex. (The one time we tried without, I sprung a leak that managed to shoot my husband in the eye. Hilariously awkward, and not much of a mood setter. Lol.)
Also, nursing pads! If I had known how many I would go through, I would have gotten some cloth ones to rewash rather than buying box after box of disposables.
Good luck with nursing! :)