your level can have an unlimited range, one important thing is you have to use LOD (level of detail).
Also you will have to make sure to remove everything that is not in the actual view.
Who told you that?
have a look at variables and trying to store coordinates larger than that.
If you need it larger than -1048577.999 to 1048576.999 try scaling down your models changing camera arc, possible but probably unneeded (simulate streaming).
Otherwise just load different levels to get what you need.