Rules:
*You can teleport into and out of it at will
*It has a couple of plug sockets and can connect to internet from the region you teleported in from
*You can take objects and people with you
*As already stated, it is (3m)^3 (3m*3m*3m). The walls are plain plaster with a light in the middle of the ceiling. The pocket dimension is topologically toroidal, so if there weren’t walls and a ceiling/floor (which you can actually destroy) you would loop if you went more than 3m in any direction. Gravity, then, is artificial and can be altered to anywhere from 0 to 2g from a dial on the wall.
Edit: additional specifications
*You can only teleport out to where you teleported in from.
*Time proceeds at the same rate inside the pocket dimension
*There is an eject button for those inside to get out if something happens to you
So does it just stay at the same point relative to the gravitational center of Earth? What about the day/night cycle; does the Earth keep rotating under it? And how big a mass is needed to lock it in place? It’d be pretty sweet for long plane trips if it traveled with the plane.
It stays relative to a reasonable thing, how humans would expect it to. If you were on a plane, it would stay relative to that, if you were on the ground of on the moon, similarly.
The position is the same as if you handt teleported in the first place.