Number of pages for a Cover-letter to apply to postdoc position
Please follow the link. Everything is properly mentioned as follows.
http://www.nature.com/scitable/ebooks/english-communication-for-scientists-14053993/118520525#headerAndCitation

The structure of a letter written for an entry-level position will differ from the structure of a letter written by a master's or Ph.D.-level researcher.
Typically, a more advanced position will require a lengthier cover letter.
In all cases, however, remember that your readers are extremely busy. Therefore, keep your letters from extending beyond two pages, unless the job advertisement specifically requests more details about research or work experience.