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
You can’t bring in more stuff than would fit, as it just wouldn’t fit. If you did, it would become a black hole. If the space you were going to pop out at would be occupied/lethal, you instead pop out at the nearest nonlethal space. And yeah, voids underwater would be cool.
It wouldn’t be a black hole, it’d be like pushing concrete into a slab of concrete. It just would take an insane amount of force to compress, and you’d have no leverage
Conversely, what’s the range for taking things in and out?
What if you brought in extremely pressurized liquid air? Could you just take 1cc and release it next to someone’s head? If you took open cones of tungsten in, you could probably abuse it to shoot them like a bullet
What about thermal properties? If the space wraps in on itself, there’s nowhere for the heat to dissipate. You could fill it with extremely high temperature plasma, then take out a little at a time to melt/explode almost anything.
That all depends on the distance - you could fill it with thermite and burning magnesium for a trigger to fill it with extremely hot pressurized gas, but if you have to touch to take it out you’d destroy the body part that took it out… You could sacrifice a finger to blow up someone’s head, but ultimately your power would stop at being a suicide bomber
Personally I wouldn’t do any of that. It’d ruin the outlet