Every role is different, and therefore every CV is different. However, we believe as a rule-of-thumb that one A4 page is the right amount of space to include all the relevant information.
This may change over time as a highly developed role may require detailed explanations of your skills and experience. Generally, however, an employer could have to sift through thousands of CVs a week and so anything that is too long may get automatically overlooked. Quality over quantity is a good motto for CV writing.