CVG: How big is Super Mario Galaxy?

While fingers are crossed in the hope that Super Mario Galaxy is at least as big as Super Mario 64 (which had 15 main levels, and numerous smaller ones), it is reported that Mario's next quest will take him through six worlds.

CVG suspects that this accounts to the environments of the main worlds in the game (so far we've seen fire, water, sand, haunted houses and Bee worlds) as well as more generic space environments with smaller rock platforms, and what appear to be bonus platform worlds - similar to those in Super Mario Sunshine.