The longer the IT project and the bigger the team, the more likely it is the project will fail.
Make the project too long, and there's no sense of urgency. Make the team too big, and it's too easy for team members to hide behind the herd.
No matter how big the effort, you can break it up into projects that don't violate the 7-by-6 rule: 7 core team members, 6 months or less. That's projects, not phases. The difference: If a phase goes long, the team can figure they'll make the time up in the next phase. If a project goes long, it's just plain late.