i just did a few quick dead reckonings.. my main base is 90 2m lengths wide and long so 180 meters takes up almost half the height and width of the minimap..so minimap is a bit larger than 400 meters by 400 meters.. i then went to undiscovered area on my seed.. and it appears fog of war dissapears around 50 to 80 meters (terrain was uneven :-) )
so hypothesis time (guessing more likely) haldors bubble is a sphere/circle... so too heat from a fire so im guessing distance from "mistlands" will be a sphere. if the distance they are quoting is 300 to 500 meters then imho that means " not "revealed"on or by your minimap!". i deliberately didnt explore any mountains.. only just spotted their edges prior to ice caves update.. it appears the caves may have seeded despite me having circled but not entered two mountain areas..

as to your question about world size..i am unable to even try to answer that with the limited "discovery i have made in this seed"

but in my continuing to guess mode: here goes !! :-) if the minimap were 500 meters x 500meters an explorer would need only travel 20 minimaps north and 20 minimaps south to guaranteed hit one of the "poles" If the world is indeed a 40 x40 minimap world equalling 20k's x 20k's