In general, NEVER neg a woman on body features. That is the equivalent of a woman making fun of your p3nis. Only the most beautiful, gorgeous women, who are displaying a bitchy, superior attitude can handle a physical neg.
A neg should address character issues and choices (choice of clothing, for example). Saying something about a woman's physical attributes that she was born with is wounding her deeply and she'll never forget it.
Many greenhorns have no idea under the sun when and how to use a neg. OP, do you know when to use a neg and why?
The plainer and more average a girl is, the less she can handle a neg without being deeply hurt. You don't want to deeply hurt them (except in rare circumstances which I'll explain below). You want to SUBTELY bring doubt about herself into her mind. You don't do it off-the-cuff, just because you think you're "supposed to". Rather, you do it for very specific reasons, and only when necessary.
Remember, a good rule of thumb is that a neg should have two possible meanings. You are making a comment about something about her, and she is unsure whether you mean the worst possible interpretation or not. It destabilizes her, shakes the ground upon which she normally walks.
If you find yourself considering making a blunt statement about her physical attributes, you're not in neg territory. You're in mis-calibrated insult territory, and she will perceive it as such, and rightly so.
The only exception that I can think of is if some arrogant b!tch starts in with you (whom you are not attracted to and have no intention of dating). Telling her in a detached way that she should spend less time worrying about you and more time on the treadmill is fair in those situation IMO.