Answering shortly, it's €1.
Our current bid allocation system created so that it would be the most efficient and fair to all our investors. There is no minimum bid size setting for the products, as it is based on the demand for a loan, and ensures that every investor who wants a part of a loan can get it. For example, if the demand on a loan is high, it is likely you get smaller bids into such loans. It always tries to use your bid size set, but it also needs to take into account other investors bids. If there is less competition, your bid can get in full (with the bid size set), otherwise, it tries to make any bid possible to fit in. The minimum amount for one bid is €1.
You may also increase your chances of getting higher bids into loans if your maximum investment per loan is higher.