Small demos are just that... what happens when you go big? What about the value plan? Will it still be good when large? These questions and more are considered as I create a big painting from a 6x8 study. A benefit of class is that I bring you along in the studio while I work on my current projects, not just 'teaching demos' so you get to see how I work, or re-work ideas and art.
When creating art think about your intent... the feel of the artwork. Often this can help you decide the main value collection. Try to have only 3-5 main values. Review the Digital U videos for more on values and planning 'mass shapes' of 'light/dark'. Don't make the mistake of thinking... 'oh I've got this' and move on to the art. This is the time to make a study, even if the Notan or mass-study is large and on your painting in blocky shapes.