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
No, it’s relative to a more sensible object than the sun, of Sagittarius A* or whatever you were imagining. Probably something like the closest planet/moon to you is what it’s actually relative to.
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.