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
I think I’d make various uses of it, but I think I’d mainly use it as an “at-sea cabin.” Rig it up with a small bed-over-desk type situation, one of those little dorm minifridge/microwave deals, and stock it with tools/supplies etc. So basically I have a weightless camper van that’s always with me and never needs parking. Hell certain city lifestyles where your “home” is where you sleep and shit and you spend the rest of your day out and about, it’d be all you need. Only problem is no plumbing.