I have yet to find a description of the file size threshold at which WP8 Onedrive requires Wi-Fi. What is this threshold, and how can I change it? I am not opposed to hacking if necessary.
As far as I can tell, there is no published size at which Wifi is required. If you look here though, there is a setting you can change to determine whether only Wifi is used for full quality images or if cellular can be used as well but the images could be reduced in quality.
Before they introduced this setting, you HAD to be on Wifi to upload.
UPDATE: In the comments, Joe says it's 20MB. Joe works for Microsoft so I'm inclined to just take his word for it.