Not necessarily twice the space - in most cases you'll need the same or less free space that your table/partition is currently using, but in some cases it could even be more.
It depends on how sparce your blocks are currently filled and also whether you change your storage parameter for the rebuilded table/partition during the move operation.
And this "twice the space" is probably not documented anywhere for this exact reason - because there is not one exact answer to your question. It could be less, it could be more, it could be the same. But the whole thing is very well documented in Concepts manual - if you understand the storage structures of the oracle database (rows, blocks, extents, segments) you can allways predict how much additional space you'll need for MOVE operation for each particular case.
ASCII a stupid question, get a stupid ANSI
24 hours in a day .... 24 beer in a case .... coincidence?