A précis is an intelligent summary of a long passage. It aims at testing your understanding of the passage. As it is a summary, it is always shorter than the original passage. It expresses only the main theme as concisely as possible.
There is a great difference between the summary and the precis or the substance. The precis or substance is the central idea of a passage. The difference between them is not only in degree but also in kind. There’s no hard and fast rule about the length of the summary, but so much can be half of the original passage.