I think we are expected to choose the "Default Merged World" option. I believe they are pre-counting the current server populations into the New Worlds. In other words, if everyone from the previous worlds merges to the new world, it is listed as full.
The full status is then just there to discourage people from a non designated world to choose that new non designated world.

Does that make sense? I'm not sure. Hopefully we get some clarification on Thursday-Friday.