Blackrock Depths is the largest 5-man in the game, if not the largest instance, and it boasts over 40 bosses (that's mostly counting encounters, though -- you wouldn't count The Seven, for example, as seven different bosses), including lots and lots of optional bosses and even a holiday boss. Ulduar, by comparison, has about 14, and Karazhan is about that same size (though that depends on how you count random bosses, like the Opera Event). No matter how you slice it, 31 raid bosses is a ton of bosses to go through -- Icecrown could be a return to a really epic, large-scale instance.
Of course, there will likely be wings involved (Naxxramas' Quarters have worked out pretty well, I think), and with the new changes to raid lockouts, Blizzard no longer needs to squeeze the raiding experience into an average of two or three nights a week (which is what it seemed like they were usually aiming for before). If yours is the kind of guild that likes to clear everything in one night, though, you might want to start freeing up some time now.
Update: GC now says he didn't mean the instance would have 31 bosses, he was just throwing in some ridiculous number to prove a point. Our question: why did he say "yes, IC is that big" if he didn't mean it would be that big?