If you have managed and implemented other CMS systems, then it is more than likely that you'll understand how to manage a Sitecore project. The fundamentals of Project Management remain applicable for all web projects but there are some Sitecore specific considerations that you have to keep in mind.
Managing large Sitecore implementations is irrespective of the Project Management methodologies. You could use RAD, Agile, SCRUM or the traditional Waterfall approach and Sitecore fits well with all different methodologies.
For large implementations, you need to identify major risks. Risks could be technical risks such as scalability, new management or technology processes, using untested modules and others. There could be business risks too, like resource availability and aggressive schedules that dont provide sufficient time for testing or changes in requirements and change-related risks such as internal adoption of the system for implementing a good Sitecore website.
An important factor in deciding the schedule and timeline also is the availability of experienced Sitecore and .NET developers. Most .NET developers can easily pick up Sitecore but the project schedule should provide sufficient time for developers to get trained and certified on Sitecore and take into consideration the learning curve associated with it.
Just like any other web CMS implementation, Sitecore needs to be evaluated from all different perspectives. A lot of planning is involved in the Inception phase of the project. The earlier this is done in the Project lifecycle, the better chances there are for the project to succeed.