The One Where Chandler Takes a Bath - S8-E13
Corrected entry: When Chandler is in the bath, Phoebe congratulates Monica on his 'size'; but in an earlier episode she tells Chandler and Monica that she looks when men are on the table and Chandler says 'You look? You massage me' and she laughs and says 'I know' which suggests something else.
Correction: In the massage episode she could just be trying to psyche him out, Phoebe style. With Phoebe you can never be sure.