DMP Plans sizes depend on the funder and other requirements. Normally for grant applications, it is an overview and is one to two pages long.
A number of good guides to making Data Management Plans (DMP) exist online from various sources. A few of them are listed below:
New from The Office of Science and Technology Policy(OSTP). On August 25, 2022, the new OTSP guidance calls for federal agencies to make taxpayer-supported research publicly available immediately, doing away with an optional 12-month embargo. It also requires the data underlying that research to be published. Federal agencies have until December 31, 2025, to institute the guidance. The full memo and the press release are linked below, along with articles discussing the impact of the proposed change.