There is a straightforward strategy that allows you to fit your subject on the page without running out of space.
I start every single observational drawing this way:

  1. Decide what you want to draw and find your frame or cropping by holding up your fingers or an actual viewfinder.
  2. Begin your drawing with four placement marks: top, bottom, left, right. These are the extremes of your frame.
  3. Double-check the height to width relationship on your subject and adjust, if necessary.
  4. From now on there is just one rule: you can’t draw anything outside these extremes!