Gather lots of photos, paintings, frame grabs, etc to help us better imagine the characters in your film.
BROAD REFERENCE: Begin by searching as broadly as possible. Look for "larger than life" characters from live-action films, rejected character designs from "making of" animation books and illustrated characters from picture books and Graphic Novels.
When casting your story with characters consider the rich resource of live-action film, especially from the golden age of 30's, 40's and 50's Hollywood. Many of the characters portrayed were larger than life and translate beautifully into graphic character design.
Try following up on artists whose designs lend themselves to your story and your own visual sensabilities. At this point you want to feed the design process with as many possibilities as you can, taking advantage of as much work as possible that has gone before you.
NARROW REFERENCE: Once your design direction is established, you'll want to choose only designs that have specific contributions to the character you are searching for. It may be body type from one, hairstyle from another, or the attitude conveyed in a third. The images compiled at this stage should result in a kind of a composite image of the character you are hoping to design.