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
Man, the places I could hike without having to carry a pack! Wouldn’t need to plan for resupply or anything. Would have communication access from anywhere in the world.
You could use it as a kickass home gym where you can dial up or down gravity depending on what you’re doing.
Smuggling would be a whole lot easier if you went that route. Transporting all sorts of things really; you only need to get yourself to the new location.
Could do some very amazing magic tricks since you can disappear (and swap props or outfits or whatever).
With gravity turned down you could presumably build up some momentum and launch yourself places. Not sure how you’d stick the landing unless you can control which way is “up” as you teleport to the room.
Edit: Just realised this idea is kinda similar to an ability from HunterxHunter