WAYS TO AVOID ANSWERING: DOES THIS MAKE ME LOOK FAT?
1. "It flatters your figure."
This works even if your girlfriend really is fat and even if her outfit is very unflattering. How's she going to argue with this logic? Try it and be amazed.
2. "I think you look sexy."
This one is probably closer to the truth than our other ideas; after all, would you still be with your girlfriend or wife if you didn't find her sexy at least some of the time? Phrasing your response this way allows you to evade a direct reaction while letting her know you still find her attractive. It also doesn't specifically address her clothing preferences either -- just her.
3. "You always look beautiful, amazing, etc."
Most people do not correlate being fat with beauty, and this will let her know that no matter how she looks or what she wears, she's still sexy. This one might not work if your woman's just trying to start a fight, though, in which case you're screwed every which way despite how many of these lines you toss at her.
4. "It's nice, but I know what would look better."
This way, you step around the question by saying it looks nice when it might actually look like crap, and then you may want to help her find a combination of clothing that you think makes her look really good. This is also a great opportunity to take her clothes off for her and have hot, spontaneous sex on the floor.
5. "You would need to be fat to look fat, and you're not fat."
while a little more long-winded and redundant, this is the shortest possible way to say this. Again, this does not attend to her attire-just her. It also supplies a better explanation than "no" without the brevity and open-endedness of "no." Women love when you can explain yourself succinctly... a good skill to develop in the event she catches you banging your secretary and wants to know why.
6. "You make that dress/outfit look good."
Do exercise caution when saying this one because a woman may interpret it as you saying her clothes (and thus her taste in fashion) is bad and she'll start griping. That's why it's at the bottom of the list; not as good as the last five choices, but still a way to avoid answering, "Does this make me look fat?"