Just looking at the map, a slightly better/more conventional measure of Stockholm might be Stockholm municipality[1] + Huddinge, Sundbyberg and Solna, which gives an area of 350 km2 and a population of about 1.1 million.

[0]: https://en.wikipedia.org/wiki/List_of_metropolitan_areas_in_... [1]: https://en.wikipedia.org/wiki/Stockholm_Municipality