I'm a hybrid. The LOML and I will decide I am going to build something be it a dresser, bed, cabinet, etc, and then we go picture hunting. If I can find a plan we like, I will buy it, but I have not had a lot of luck with plans. Too many times either the joinery is not as good as it should be, or more likely I suck at that type of joinery.
If we don't find a plan, then I will do drawings laying out how I will make it from the picture. For dimensions, I try to stay pretty standard and rob them from other plans/books/magazines that I have. One that I use a lot for dimensions is Thomas Moser's measured shop drawings for american furniture. Historically, I would get out pencil and paper and do rough drawings, but I would end up forgetting or missing something. Now I pull up sketchup and do it all. One thing I have noticed with me doing sketchup is that I at times make the joinery more complex than I need to..