I needed to make all the videos 480 pixels high, and scale the width proportionally.

That being said, i came up with this PHP script that i call from inside a bash script:

#!/usr/bin/php
<?php
$cmdWidth = 'midentify '.$argv[1];
$finalHeight = $argv[2];
exec($cmdWidth, $output);

foreach ($output as $value) {
    if (strstr($value, "ID_VIDEO_WIDTH")) {

		$width = parseResult($value);
    }
    if (strstr($value, "ID_VIDEO_HEIGHT")) {
    	$height = parseResult($value);
    }
}
$frameSize = $width / $height;
$finalWidth = $finalHeight * $frameSize;
echo getMod16(round($finalWidth)) . "x" . getMod16(round($finalHeight)) . "\n";
function parseResult($line) {
	$v1 = explode("=",$line);
	$v20 = $v1[1];

	//var_dump($v2);
	return $v20;
}
function getMod16($number){
	while (fmod($number, 16) != 0) {
		$number ++;
	}
	return $number;
}
?>

This script assumes that you have the “midentify” utility (wich comes with mplayer) installed in your path, and expects 2 arguments, being argument 1 the movie you want to resize and argument 2 the actual height you want to reach.
That being said, suppose you have a movie that is 848 pixels width and 480 pixels high and you want to re-scale it so it fits inside a 400 pixels high space, you would call the script like this:

./resizer mymovie.avi 400

The script will output this:

720x400

You can use later this value as an input parameter for ffmpeg for example.

Before somebody asks, the reason because i used PHP instead of BASH, is simply because bash does not supports floats.