The easiest way is to look for a boot animation you like that uses 480x800 resolution. Then delete /system/media/bootanimation_virgin.zip, rename your new one to bootanimation_virgin.zip, and toss it in /system/media. You will need to change the permissions to match the original one.
Make sure you have a backup and a way to get into recovery without booting to Android. If your only way to get to recovery is through Android I would not mess with this because you could end up with a brick if the boot animation does not work or you make a small mistake.