Let's tackle your questions from the last to the first:
3) Is there a reason why it seems like there isn't a lot of attention dedicated to this topic specifically?
Yes, there is. The whole thing of project-management processes is about solving problems like inner-team communication, work sharing, team conflicts, responsibilities, ... the complexity of all these problems explodes with increasing team size. Hence, for a small team you there's just no need for specialized methodologies.
The only problem specific to small teams is probably missing manpower, and the only solution to this is increasing the team size ;)
2) How does a single person manage his or her project most effectively?
There is lots of methodologies out there, for how to manage your individual work. Some examples are Getting Things Done, Manage Your Workday Now, The One Minute TODO List, Zen To Done, Personal Kanban, and many more... This is not what you'd call project-management methodologies, but it's still management methodologies. Usually those methods try to be as lightweight as possible. This is because many problems from project management just don't appear in self management. For example, you usually know what you did yesterday and don't need to communicate it to yourself. Team starts with two members.
1) Has there been any work done to develop a methodology geared for groups of this size?
To the best of my knowledge, there is no work targeting such group sizes explicitly. However, that does not mean that all is lost. The ideas of project management processes that target larger team sizes also apply for smaller teams. Only you would apply them in a more lightweight fashion or even drop some activities, to reduce overhead. Let me give you an example:
The Scrum process includes short daily status meetings, called daily standups. In a team with 7 member you would schedule these meetings to "every day at 10am", to make sure they really happen. In a 2-men-team you could just as easily exchange you status during lunch or in the coffee break. It may even be that you exchange your status anyways, because your sitting in the same office and talk to each other during your workday. The general idea of a regular short status exchange still applies, only it's done in a more reasonable fashion, considering the project's circumstances.
Conclusion is that you can adapt most, especially agile, project-management methodologies to really small team by looking at the ideas and adapting their realization to the situation at hand. That's what most of these methodologies tell you anyways.
