I personally find that when you pick a subject that really interests you, that it helps the project move forward even when things get slow. That being said, I would pick a handful of aircraft that interest me, then start searching for plans of the appropriate size that you would like to build.

I have several books of aircraft of the world I thumb through when looking for a new subject. I have also attended a full size airshow and found a subject that caught my interest. I like scale and prefer Canadian registered aircraft or WW aircraft flown by Canadians. The story and research is a large part of the fun as much as building sometimes.