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
It becomes my house. Now all I have to worry about is food, water and a few incidentals. To shower, I could probably exploit the geometry for endless water pressure instead of using a pump, then I’d just need a little heater and a filter of some kind.
The first thing I do, of course, is dick with the gravity dial. See how low I can get it before I lose my lunch, see how high I can turn it and still do everything I need. Maybe I stick something heavy to the side of the dial so it turns itself and so on.
Maybe to raise the rest of what I need, I’ll start a moving company.
The weird geometry could also have some engineering uses that are pretty unique. For example, you could make a magnetic bottle for plasma that doesn’t leak as it wouldn’t need ends, or a laser in a frequency of light that’s hard to reflect.