Depends what you mean by large - typically its the rental period, i.e. one months rent if paying monthly, although if paying in 6 month blocks then it is usually negotiated down to about 2 or 3 months.
A friend of mine had been looking at a small unit in the city center were the Landlord was looking for a couple of thousand euro up front plus a personal guarantee. A lot more than the actual rent it's self.