You will lose everything, except your character's experience points.
Edit: this is it how it works when switching between the island, or the center as an example. I have not tried it yet going to a PGM, so don't hold me to it!
I don't believe this to be possible. From some of the settings I see in the command line, you can only specify biome sizes using coordinates. This would imply the actual map size is set to a specific set of cords.