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
Hmmmmmmmm, now, how much energy does the box have to generate that constant 2g of acceleration? In this hypothetical the box appears to have an infinite amount of energy to generate that force though…
Yes it seems to have infinite energy but the throughput is limited to 2g of acceleration, unless you give it infinite time as well it will not reach c, though it would approach it.
Doing some calculation the final speed of 33kg, falling in 2g, for 70 years, without friction is “only” 99.77% the speed of light.
Edit: Forgot to convert the 0.9977c to percent.