I read on the website that there were only 5 major zones. Is that it or are they really large zones broken up into different subsections. I hope the game world is pretty huge to keep it from getting mundane.

The zones are:

Canada
Western Desert
Millennium CIty
Monster Island
Lemuria

I'm not aware that there are any more. You wont see Monster Island or Lemuria until you'e 30+ so for the first 30 levels its just the 3.

Levels 5-13 you're in canada or the desert - if you do both zones you can get to about 16-17 before hitting MC
MC up to about level 20 then its off to the desert for a few levels and then to canada then back to MC around 25 for a couple of missions and setting up your nemesis, (then you can go to stronghold in the desert but the game wont tell you that - which is a shame because its the best instance in the game imo) then back to canada til about level 30 then its off to monster island.

The game world isn't that huge and there is only one main hub per zone (there are smaller hubs to get quests from but they don't have a powerhouse or crafting tables or shops. Monster Island at 30 is a great change of scenery because by then you'll really need one.

The only thing NancyJames didn't mention is that each zone is about 3.5 times larger than the typical COH zone.