I made script that allows members to upload an avatar for the forums but i want to add something to it that will check that the image is 64px by 64px and that the file size isn't too big. How can i do this?

If you have the GD libraries available (a lot of hosts already do), you can use them to do this:

PHP Code:

$image = imagecreatefromjpeg($filename);
//or
$image = imagecreatefromgif($filename); 


^ This will get you a handle to the image, you an use the filetype($filename) function to determine whether it is a gif, jpeg, png, bmp or whatever, and call the right creation function ( a complete list is here:
http://www.php.net/manual/en/ref.image.php )

imagesx($image) will get you the width and imagesy($image) will get you the height in pixels.

filesize($filename) will give you the filesize (in bytes I think).