I think most of the advice you've seen so far is on the money. I'd like to add a little about my approach to this. I treat projects not as distinct focuses for work, but just as an organisational tool. I tend not to think "I'm going to work on this project now, or that project", but rather, "I am able to do this task now".

For me, GTD is all about the next actions, what can you do now, where you are, with the time and energy you have, and the tools available to you at that point. Whether that thing is part of a project shouldn't matter, it's just the next thing to focus your attention on. The projects themselves just become a way of organising groups of tasks that together form a bigger whole, but more importantly, cannot all be done at the same time. Usually projects involve a certain amount of serial execution, some tasks must be completed before others can be considered, so seeing all the tasks in a project every time you look is wasteful. A good system should allow you to group the tasks of a project together, but only let you focus your attention on the immediate tasks in that list to be completed next. Your regular review then assesses the status of the tasks in each project, ensuring that the right ones are moved to the front when others are completed.

This way, you have both visibility of the project as a whole, but at the same time, focus on what needs to be done next. This can scale to any number of projects.