Read this strip at gocomics.com.
Lucy's not been an overly loud character so far, but the sheer size of her voice becomes a distinctive feature in the coming years. (But not before another character becomes known for it....)
This strip depicts Lucy in four different sizes. One of the interesting things about comic strip art, I think, is that it is almost always depicted in set sizes. Considering how fluid is the medium, it seems weird that characters are always produced at the same sizes each week.
There is at least one good reason for this. Comic characters are drawn using pen lines, and those have set thicknesses. If you reproduce a comic character, especially a black-and-white one, at a larger size but without using a thicker line, they tend to look a little funny, like they're "lighter" than they should be. The opposite happens if you draw a character smaller but use the same size line. And comic artists, who must be able to precisely reproduce characters, probably have concerns about their ability to do that with different thicknesses of pen.
If you draw the character at its usual size and just resize it using printing equipment, however, it will stick out if it shares a frame with other objects drawn at normal size.
The solution, as Schulz demonstrates here, is to redesign the character at different sizes, simplifying it the smaller it's drawn. Lucy gets less detailed the further away she gets from the viewer.