Update/debriefing/discussion:
The take home message I am getting from feedback is the following:
When confronted with this scenario in the future- DO:
1.Look up from the computer.
2.(Not stated, but implied)- Look at the dress and make eye contact BEFORE answering.
3.Say "That looks GREAT honey"
DO NOT:
Say "Your arse looks big because your arse is big."
Fortunately, last night's dinner was great. It was a beer/food pairing and if there were any problems, the 9% ABV concentrations of the last two beers put an end to any concerns.
As a final FWIW, I did have some success after my moment of hesitation to pull a comeback move. As she went in to change, I put on a 20 year old Sport Jacket that came from the era of "flashdance." I popped in, and enthusiastically asked her if it looked "Okay" and that I had other sport jackets if it wasn't any good.
She told me to "F#$ off". -so I think it worked.