A Hubble volume is a spherical area, that would surround an observer from a single point. Beyond the horizon of the Hubble volume, objects are receding from the observer, faster than the speed of light. A Hubble volume's size is defined as as the region within which galaxies are receding slower than light. This is due to the Universe's expansion. That being said, a Level I multiverse, would be a kind of, extension of our universe. Let me explain. This kind of multiverse, would be infinite in size. That being said, if it is infinite in size, it would have to contain all possible configurations of Hubble volumes. There are infinitely many. That being said, there must be one, that looks just like ours. Max Tegmark postulates that one identical to ours, should be at about: 10^10^115 meters away from ours. If there is infinite space, than there is an infinite number of Hubble volumes, some even identical ours. The Level I multiverse will include your doppelgangers.
Indeed, this is a speculation, not based on modern physics, however, one two premises: that the universe is infinite and that it is rather uniformly filled with matter.
Different Hubble volumes in a Level I multiverse.
Level I parallel universes are simply universe-sized parts of our space that are so far away that light from them has not had time to reach us yet. There are no physical boundaries between the neighboring universe, nor would we be holding a special place in the multiverse.