How far is a yard on screen?

Here is an awesome visualization of the distances, courtesy of diablo.incgamers.com. Note the dashed lines that indicate different display ratios.

So 10 yards gives you a radius about the height of your character, while 40 yards gives a circle that reaches just about to the edge of a 4:3 screen (note that you can see further in front of you than you can in back).

Here's the larger discussion around the creation of this image.