My definition of a project tends to be based on whatever makes the tasks in OmniFocus (my GTD tool) flow better, and _that_ tends to be based on breaking the bigger project into subprojects where the tasks run sequentially, instead of concurrently.
For example, I _could_ have one "get painting done" project:
Project: Get Painting Done
Tasks:
Call Painter A
Call Painter B
Call Painter C
Arrange walkthrough with Painter A
Arrange walkthrough with Painter B
etc.
But I'd rather see a next action for _each_ painter, because there's no need for, say, "Follow up on Painter A's Quote" to wait for "Call Painter C". And I realize that, yes, I didn't need to list all those tasks up there - I could, and perhaps should, list precisely one next action. But I like my system to remember thjings for me, such as remembering that, oh, yeah, I was also going to talk to Painter C.
So I might have the following group of projects:
Project: Get Painting Done:
Waiting For: Completion of project Get Painter A Quote.
Waiting For: Completion of project Get Painter B Quote.
Waiting For: Completion of project Get Painter C Quote.
Project: Get Painter A Quote
Next Action: Confirm walkthrough with Painter A
Project: Get Painter B Quote:
Waiting For: Painter B quote.
Project: Get Painter C Quote
Next Action: Call Painter C
Those three projects could be a subproject of the main "Get Painting Done" project, but purely as a matter of personal taste, I don't like that - I prefer parallel projects that refer to each other with a "Waiting For".
Editing to add: Of course, I could break up the project differently, with more of the structure in the project support materials and less in my GTD system itself:
Project: Get Painting Done
Waiting For: Get Painting Quotes
Project: Get Painting Quotes
Make spreadsheet of candidate painters, with space for walkthrough dates.
Repeating, daily: Call at least two candidate painters, requesting walkthrough and quote. Stop when four painters are scheduled for walkthrough.
Waiting for: Give painters two weeks for walkthrough and quote.
Repeating, daily: Analyze one quote. Stop when all are analyzed.
and so on. The walkthroughs themselves wouldn't even be in this project, they'd be hard calendar items.
Gardener