The installation has two separate Size categories: its inside category (purchased normally), which determines the structure’s interior space, and an outside category, one or more size categories smaller. In essence, the installation is larger on the inside than on the outside! So a small house, for example, might contain the space of a huge castle on the inside. The GM may even allow size categories beyond Awesome, with each additional category doubling size; expensive HQs could be pocket universes! Pay the cost of the larger size, plus this feature, which lets you set the exterior size at any smaller category.
In general, the exterior dimensions of the installation cannot be smaller than a miniscule structure, about the size of a closet or phone booth (or, say, a wardrobe or police box), large enough for an adult human to pass through whatever serves as the base’s entrance. Headquarters that have no “exterior” structure, such as an extra-dimensional fortress accessed by a magical talisman, do not have this feature, but instead have things like Dimensional Portal, Isolated, Sealed, and the like.
D20 Hero SRD Installations Article- http://www.d20herosrd.com/7-gadgets-gear/installations