“That dress makes you look fat,” is an opinion statement, not a fact statement. “Fat” as an appearance quality, rather than as a type of flesh, is a matter of opinion. Then there’s the issue of “the dress makes you ...”. If the viewer already considers the questioner “fat,” the dress isn’t doing anything. As the Irish joke says, “Do these jeans make my butt look big?” “No, chocolate cake makes your butt look big.”
Therefore, this situation is not one in which “lie” is a useful verb. It’s a question of determining the subtext of the question. Some options:
“Will you reassure me that I look okay for the planned event?”
“Is this dress the most flattering to me of my available options?”
“Do you think I’m fat?”
The answers depend on one’s character. A decent husband would reassure his wife. If the dress has a rip, has a stain, looks awful with those shoes, etc., he would tell her. A perceptive husband might say, “That one’s okay, but you look really slinky in the blue one.” An unutterable cretin would say, “Yes, I think you’re fat,” and flatter himself that he never lies.
Soon to be EX cretin; I think!