The program itself contained all the souls from Limbo/Hell, as seen during the ending when messing up the last of the game's security code caused the release of all the imprisoned souls. The game itself is obviously not a parallel universe in the same sense a diverging timeline is (a variation on the main universe), but is instead the realm that contains all souls that didn't make it to Heaven. Causing its destruction sends Theodore back to the living universe, right in the spot where he died. The cabinet itself and the mysterious arcade Theodore finds himself in are part of the same reality, as the game's collapse resulted in the destruction of both.
Antvasima wrote: Is the size specified at some point?
Aside from it being a separate reality? Not that I recall. It is just shown to be where everyone who doesn't make it to Heaven ends up. However, I can skim through the game again to see if anything specific is mentioned.
Matthew Schroeder wrote: Cause he is the programmer / developer of the game, so the entire reality of the game is just a code to him.
This is true. Even when "fought" during the final escape of all the imprisoned souls, the Satan present is just an avatar, and his destruction has zero effect on the actual being. Everything in the game world is just that, to him. A game.