"Zij heeft een grote neus."
Translation:She has a big nose.
It has the same idiomatic meaning in both languages. Grey236 only omitted om, so in Dutch the talking is not only figuratively happening behind someone's back, but also around it (i.e. avoiding the subject notices that the talking is happening): zij praten achter mijn rug om.
Ah ok. :) I don't think there's a Dutch equivalent, at least right now I can't think of an idiom that means (almost) the same. One could use the English phrase, but that definitely wouldn't work for all audiences. I guess quite some Dutch speakers would have trouble understanding it, idioms are difficult for non-native speakers. Some Dutch speakers tend to literally translate Dutch sentences to English, which gives sentences like: this time I see it through the fingers, the Netherlands is a country of undertakers (a former prime minister meant entrepreneurs, ondernemers in Dutch).