How should a Christian respond to "Does this dress me look fat?"? If your wife asks, "Does this dress make me look fat?" And you think it does, but you tell her, "Of course not, honey!" Is that a sin?
A successful marriage relationship will need a full measure of agape love.  Here's how Paul defines it with particularly relevant words bolded:


  Love is patient and kind; love does not envy or boast; it is not arrogant or rude. It does not insist on its own way; it is not irritable or resentful; it does not rejoice at wrongdoing, but rejoices with the truth. Love bears all things, believes all things, hopes all things, endures all things.&mdash;1st Corinthians 13:4-7 (ESV)


Let's take these in reverse order:

Rejoices with the truth

Presumably, the husband does not think he is telling the truth.  By this standard, lying to your wife is not loving.

It is not arrogant or rude

However, if the husband said exactly what he thought, that would be rude (and possibly arrogant):


  It makes you look like a pregnant cow.


Honestly, given the choice, it would be more loving to lie to your wive than to be rude.

Love is patient and kind

But there's another way.  The patient thing to do is to ask questions or let the wife down easy.  I've found that saying things like "I wonder if it would be a good idea to accessorize?" communicates that something is not quite right, without being rude.  (I don't know what accessorizing means exactly or how it could help, but it shows my wife that I care.)  Another thing that works for me is to spend "too long" appraising the outfit.  After a minute of looking and contemplating, my wife will get the hint: something isn't right.

If we are about to go out and I think there's a good chance she will be embarrassed if someone posts a picture of the outfit on Facebook.  The kindest thing I can do is let her know:


  Dearest, I know you spent the last hour and a half getting ready to go, but I really think you'd look better in some other dress.


Then I will (Lord willing) spend whatever time is needed helping her look her best.