It should be 60 000 000 by 60 000 000 (that is 60 million) blocks.
One of the proves is that if you go to any of the minecraft 1.8. versions and type this command: "/worldborder set >59 999 999" (so any number smaller than 60 000 000 and bigger than 0) Minecraft will resize the border but it will also show you a message that the border's size was changed from 60 million blocks (to whatever you have written).
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The size of the download for Minecraft is 258 KB.
Sorry it's probaly not possible ):
The bottom of the world is the void.
It is not a seed, it is a world download. Look up dropper world download on google or minecraft forums
to play a map, open minecraft, then click singleplayer, choose the world, then click play selected world.