Mark your calendars.
Contact your local Japanese bookstores.
Be ready for it.
In Weekly JUMP #53 (that means 4 weeks from Monday and perhaps a few days earlier in some overseas locations ((and certainly earlier online)) ) One Piece Chapter 0 will be published.
Nothing mentioned about length, only that it will explain Shiki's past.
So there you have it. It seems highly likely this will be the same content published in Tank 0.
The events of Chapter 0 have already been turned into an animated...short(?)...episode(?) and will be put to limited edition mail-in-campaign 3000 DVDs. They suggest that these DVDs will be the only way to see it. I dunno about that since there's an opportunity for them to make money BUT it will probably be the only way to see it for the time being.