I have ordered ExOfficio tank tops in the past.  I wear a small, but the small was really very small, so I ordered a medium next time.  Great.  BUT, this year I ordered a medium and it is huge.  I kept it as a sleep top, but ordered another in small, and it is still bigger than last year's small, but it is a good fit.  Bottom line,  if you usually wear a size 6, go with the Small in this.  As far as quality, you can't beat ExOfficio underwear.  Fast drying, wicking, comfortable  -- great quality.  Worth the money.