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
This is a terrifying power, I gotta say. Like a dude could carry an entire armory around and deploy it Looney-toons-style wherever he wanted.
I would probably just put a bunch of shelves in it and use it as a personal EDC gear storage space. 3m^3 is more than enough space to store pretty much anything you might need in a jiffy.
There’s also apart of me that thinks putting a bed in there would mean not having to bring a date back to your place or to a hotel - but being real I’d probably never use it if I set it up like that.