If it were me, I would mix the job up among 4 or 5 design companies for a couple reasons:
1) There are a lot of really good design people here... good to spread the love on a big project
2) Sometimes people get burnt out easily on big repetitive jobs for a single client like this. And depending on how low you wind up letting people bid this down to - that time-till-burnout factor will go up as the designers start to feel bad about bidding to low.
3) Most designers have their own unique style... unless you for some reason really like one guys style enough to have it spread out on a hundred sites, you might want to mix it up a bit for varitey.
4) See #2.. but I've found that SOMETIMES, the more you give someone to do at once, the longer it winds up taking them to complete the entire project.
Anyway theres my
