Proportional study sketches from master drawings, paintings and built works of architecture are some of the best possible exercises for one's sense of proportion. I try to do them whenever I can.
Simple tools - just a pencil and sketchbook are all that is needed. Here are some examples from my travels:
Proportional study sketch by James Dougherty from the painting Portrait of a Young Man – by Hans Holbein the Younger in the National Gallery of Art, Washington DC
Julien Guadet (1834-1908), the esteemed French architect, theoretician and Professor at the École des Beaux-Arts, Paris writes in his four volume masterwork Éléments et Théorie de l'Architecture:
"The sense of proportion is the chief artistic sense. Proportions are infinite and delicate in art and are still more so in nature.
How do you recognize a friend among all the people you see, even though you see millions pass? A question of proportions alone. But proportions are so infinite and variable, that among millions of heads no two are exactly alike.
Nothing develops a sense of proportion like practice in drawing. To draw is to perceive and then express those specific proportions that distinguish and identify the model. The best draftsman is the most sensitive to proportions."
Proportional study sketch by James Dougherty from the sculpture Theseus Battling the Centaur - by Antoine-Louis Barye in the High Museum of Art, Atlanta GA
Proportional plein air study sketch by James Dougherty of Budapest's Chain Bridge - designed by William Tierney Clark