What a fun project! I painted a barn quilt for my dad's barn.. it was an 8' x 8' beast to hang!
My biggest tip is to sketch your design out on graph paper, then take a pencil and make a little dot on your shed that lines up with each intersection. That way, you can always quickly refer to the graph paper when painting. If you're working in Row 3, Column 2, for example, it's just a quick glance to make sure you're painting correctly.