Just my two cents for screenshot with a size parameter

screenshot.js

/**
 * This script will capture a screenshot of a webpage page
 * Usage: $ casperjs screenshot.js <url> <filename.[jpg|png|pdf]> [--size=800x600] [--rename]
 *
 *   QVGA  (320x240)
 *   HVGA  (480x320)
 *   VGA   (640x480)
 *   SVGA  (800x600)
 *   XGA   (1024x768)
 *   HD    (1280x720)
 *   SXGA  (1280x1024)
 *   SXGA+ (1400x1050)
 *   UXGA  (1600x1200)
 *   WUXGA (1920x1200)
 */

var casper   = require('casper').create(),
    utils    = require('utils'),
    url      = casper.cli.get(0),
    filename = casper.cli.get(1),
    size     = casper.cli.raw.get('size') || '640x480',
    rename   = casper.cli.get('rename')   || false;


if (!url || !filename || !/\.(png|jpg|pdf)$/i.test(filename)) {
    casper
        .echo("Usage: $ casperjs screenshot.js <url> <filename.[jpg|png|pdf]> [--size=800x600] [--rename]")
        .exit(1)
    ;
}


casper.start(size, function() {

    if ( size.indexOf('x') < 0 ) {
        this.echo("Incorrect format '" + size + "', need a 'x' to separate size values");
        this.exit(1);
    }

    var size_img = size.split('x');
    var width    = Math.max(~~size_img[0], 240) || 320; // minimum = 320px
    var height   = ~~size_img[1] || 240;

    size = width.toString() + 'x' + height;

    this.page.viewportSize = {
        width:  width,
        height: height,
    };
});


casper.thenOpen(url, function() {

    if ( rename ) {
        filename = filename.replace(/(.*)\.(png|jpg|pdf)$/i, '$1-' + size + '.$2');
    }

    this.captureSelector(filename, "html");

    var result = {
        'currentUrl': this.getCurrentUrl(),
        'filename':  filename
    }

    this.echo(JSON.stringify(result)).exit();
});

casper.run();

casper.start() needs an URL as first argument:
https://gist.github.com/nhoizey/4959525