What size is the rear metal brake lines on 2009 1.4tdi ?

It's metric, 4.75mm.

Thanks! 

Is it worth having a go at changing rear brake lines on 2009 1.4tdi  fabia or leave to garage.(what size connections and type of flaring needed?)

The Fabia is made in the Czech Republic from VW components, it's a completely European car, EVERYTHING on it is metric so you don't need to ask what size this or that is, the answer is, it's METRIC, so you need metric pipe, metric connectors, a metric flaring tool etc.

The fact that you keep asking the same question suggests you should probably get the garage to do the work, they have a car lift and all the right tools and you don't.