Lately, I've been doing a lot of value studies on my commute. Mainly using photos and movie stills as references. Getting values right is something I have a huge problem with, and the exercise, even though it can be a pain, helps a lot.
Dice Tsutsumi and Robert Kondo have a good tutorial on this over at Schoolism, if you would like to learn more about the subject. http://www.schoolism.com/school.php?id=30
The drawings are something like 1*2inches in size, and I think there's a point in keeping them small, since this exercise is all about simplification. Squinting and keeping it loose with big strokes is the way to go here, I would say. No point in getting lost in details.