What file size is considered large over mobile network?

What file size is considered "large" when downloading over a mobile network. For example, if a user comes to check a bunch of PDF's uploaded by an admin, at what time should (s)he be warned about the file size being large and that they will downloading / viewing over the cellular network. Do we have a standard for this?

What could be a possible warning message? From the back of my mind I came up with this:

You are about to view a file that exceeds 10mb size and will be downloaded over the cellular netwrk. Do you wish to continue? Yes/No

Thank you for your response.