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
Fair.
But even if it is relative to the earth, there would be the question of do you retain your velocity relative to the earth when you teleport in?
So moving pretty much in any modern vehicle would be a reliable way to repaint a surface of the interior with yourself, particularly an aircraft.
However, if it doesn’t, then you can reset your momentum every time you pop in or out. In that case, you can reset your movement several times mid-air so when you hit the ground you’re going a survivable speed.
Yeah that Is also an important question. With a power like this, it’s almost scarey to use because who knows what can go wrong.