I'm going to tell my wife that her breasts are saggy
How do you think your wife will react to you saying that to her?
I'm not sure. I dont know how she will act to talking about something related to her breasts.
You should not say insulting things about someone's body or personal appearance. Insulting your wife's breasts will cause her to feel insecure and lead to problems in your marriage.
My wife would be fine with it.
Even if she is normally easy going, she would be hurt by it.   Why not compliment her on something positive instead?