This is how you can get your figures on the beach (or street) in proper size and perspective, we have and innate sensitivity to the figure so it's quite noticeable when it's not right. It also works for birds or rocks or bits of drift wood. Its an easy simple way to get it right.
He made it very basic,
I just tried this with some gulls on the beach it works very well...
First get the nearest figure to a size that looks right for you. Then draw vanishing lines from the head and feet to the horizon. Use the lines to measure the figures at the points you want to place them.
Now cut out the different sized figures and fit them on the canvas. now you can pull horizontally to where they need to be, which allows you to check out the size and place for composition.