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
Who’s paying for it? Is the Wi-Fi also entering through this portal? What other electromagnetic incursions can we expect?
I guess the power companies? But they won’t notice such a small usage. Yes, that is also where mobile data/WiFi enters the pocket dimension, from the place where you teleported in.
So you’re stealing power, and there’s definitely other EM leakage that you’re conspicuously not mentioning.