I agree w amarita, you can find 'some' farang sized bikinis in the middle stands at MV.
No fitting rooms, but the lady owner will gladly in a loud voice let you know that 'XL no fit, too small for Youuuu' and that an XXL or XXXL is better for you

....there's nothing like the reality-check you get when you shop for these things in Thailand...
Unfortunately the Phuket Mermaid shops have closed in HH, can be found in Gaysorn Plaza 1st fl, or at the airport in BKK, and I belive in some shops in Pattaya, Samui and Phuket. A bit expensive, but great quality.
We are all born ignorant, but one must work hard to remain stupid.[Benjamin Franklin]