Seed a top 64 or something (like they do in most professional single-elim formats). It'll give a good mix of matches in the early rounds, but it'll ensure that the top players don't go out really early.
Either way don't full-seed and have double-elim, because that's overkill and it's a totally retarded way of doing it. They both achieve the same goal in different ways, and there's absolutely no reason to have both.
I'd say make it 512 or larger. As big as possible really - IMO what made it so awesome last time was the absolutely enormous bracket.
I think there should be consideration, at least in the latter stages, for timezones. There are time slots that are convenient for NA-EU, NA-SEA and EU-SEA, and none that really cover all three suitably. Schedule the matches for the players, but choose a time slot that considers the time zones of each player.