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 mention of external dimensions, so I am going to assume that it exists in the abstract since you mention teleportation. Like you just think about it and teleport there, not carry it upon your person like a portable hole in D&D.
Like others mentioned, a living space of some kind. I’d worry at first where I teleport inside, like hopefully not inside my mattress. Also, when I teleport back out, where is that? Where I teleported in? What if that gets replaced, like say I popped out while in an empty parking garage, but popped back in and a car is now parked where I left off? Might be nice if I pop back out anywhere I choose. Pop in while in New York, pop out in London.
Man, I’d save so much on rent. Or at least storage.
Yeah I was thinking it would be either a mobile home, or a Skyrim bag.