Useful catchall for this situation:
Say that different brands of women's clothing use different measurements for their sizes, which is apparently true. Thus a size 12 at Brand X could be a size 14 at Brand Y.
( .Yeti. El amor en los tiempos del cólera, Fri 17 Aug 2012, 13:22, 2 replies)

It is.
Bloody pain in the arse. I'm a size 6 (eh?!) in some shops and 10 in others. No sense to it.
( vix0r was not Matthew on, Sat 18 Aug 2012, 18:13, closed)

Best delivered as an expression of disbelief, without breaking character or appearing to pause to think
"fuck off are you - you're a 10 at most"

This will either work immediately, or if she's insistent on getting offended, you continue with "well either you or my ex are shopping in the wrong places then love, cos she was a 12 and you're slimmer than her"

It is vital that you then continue the discussion with other friends, as if the disagreement is already over. Don't be drawn into validating her any more than you already have.
(bblgoose, Wed 22 Aug 2012, 17:24, closed)