Try a modified process.php look here: http://www.wpdownloadmanager.com/support/topic/download-file-size-0kb/page/3/#post-8347 Try disabling more plugins, one after another.
I created another simplified process.php for 2.5.0 hope this helps some people.
overwrite to /wp-content/plugins/download-manager
watch there for fix a http://www.wpdownloadmanager.com/support/topic/download-file-size-0kb/
did you update all of your plugins? the fix is for 2.4.9 only deactivate one plugin after the other and try downloading
