If anyone finds "The other parameter will be calculated if 0 is passed as either param. " to be a bit confusing, it means approximately this:

<?php
$im = new Imagick('example.jpg');
$im->scaleImage(300, 0);
?>

This scales the image such that it is now 300 pixels wide, and automatically calculates the height to keep the image at the same aspect ratio.

<?php
$im = new Imagick('example.jpg');
$im->scaleImage(0, 300);
?>

Similarly, this example scales the image to make it 300 pixels tall, and the method automatically recalculates the image's height to maintain the aspect ratio.

Here is an easy way to resize an animated gif : 

$picture = new Imagick('animated_gif.gif');

foreach($picture as $frame){
    $frame->scaleImage($width, $height);
}

When using the "fit = true" option, the image will only scale down, but never scale up:

<?php
$im = new Imagick('1600x1200.jpg');

$im->scaleImage(2000, 1500, true); // => 1600x1200

$im->scaleImage(1000, 500, true); // => 666x500
?>

Need to resize portrait and landscape images (and convert to 72ppi)? These will fit an area of 800x600 without distorting, no matter how tall or wide.

<?php
    $img = new Imagick($img_loc.$file);
    $img->setImageResolution(72,72); 
    $img->resampleImage(72,72,imagick::FILTER_UNDEFINED,1);
    $img->scaleImage(800,0);
    $d = $img->getImageGeometry(); 
    $h = $d['height']; 
    if($h > 600) {
    $img->scaleImage(0,600);
    $img->writeImage($resized_loc.$file);
    } else {
    $img->writeImage($resized_loc.$file);
    }
    $img->destroy();
?>

Warning: this will blur your edges in possibly unexpected ways. For better control, use resizeImage, instead.