In Friends Season 5, Episode 11, Rachel finds out about Chandler and Monica dating when Rachel overhears them on the phone while Monica calls Chandler "Mister Big". Mid-season 5 is supposed to have aired somewhere around New Year 1998-1999 (as its air dates are here) while Sex and the City has first aired in 1998 and novel was published in 1997.
Therefore, the timing allows this to be a nod to Sex and the City. So, is it really a reference or just a naughty nickname Monica gave Chandler?