fix height, width in watermark code

hello frnds,
im using below code to watermark my images

<?php
// this script creates a watermarked image from an image file - can be a .jpg .gif or .png file
// where watermark.gif is a mostly transparent gif image with the watermark - goes in the same directory as this script
// where this script is named watermark.php
// call this script with an image tag
// <img src=" watermark.php?path=imagepath"> where path is a relative path such as subdirectory/image.jpg
$imagesource =  $_GET['path'];
$filetype = substr($imagesource,strlen($imagesource)-4,4);
$filetype = strtolower($filetype);
if($filetype == ".gif")  $image = @imagecreatefromgif($imagesource);  
if($filetype == ".jpg")  $image = @imagecreatefromjpeg($imagesource);  
if($filetype == ".png")  $image = @imagecreatefrompng($imagesource);  
if (!$image) die();
$watermark = @imagecreatefrompng('watermark2.png');
$imagewidth = imagesx($image);
$imageheight = imagesy($image);  
$watermarkwidth =  imagesx($watermark);
$watermarkheight =  imagesy($watermark);
$startwidth = (($imagewidth - $watermarkwidth)/1);
$startheight = (($imageheight - $watermarkheight)/20);
imagecopy($image, $watermark,  $startwidth, $startheight, 0, 0, $watermarkwidth, $watermarkheight);
imagejpeg($image);
imagedestroy($image);
imagedestroy($watermark);
?>

PHP:

now i want fix height/width(say 100px) for the output image

what and where to change in the code????

 

Change these 2:
$watermarkwidth = 100;
$watermarkheight = 100;