It's difficult to give a precise area for the "Northern Great Plains" because there isn't a universally accepted, officially defined boundary for the region.

Here's why:

* Different sources use different definitions: Some sources define the region based on geography (elevation, climate, etc.), others on political boundaries (states), and some on ecological factors.

* Overlap with other regions: The Northern Great Plains overlaps with other regions like the Canadian Prairies, the Great Plains, and the Rocky Mountains.

Instead of a single area, here's a range of possibilities:

* The U.S. portion of the Great Plains: Approximately 350,000 square miles, including Montana, North Dakota, South Dakota, Nebraska, and parts of Wyoming and Kansas.

* The "Northern Great Plains" defined by the US Geological Survey: This definition focuses on the ecoregion, encompassing parts of Montana, North Dakota, South Dakota, and Wyoming.

* Including the Canadian Prairies: Extending the region further north to include Alberta, Saskatchewan, and Manitoba, the total area would be significantly larger.

To get a more accurate estimate, you need to clarify what definition of the Northern Great Plains you're interested in.