Is there a size restriction for files uploaded using the Valence Learning Framework APIs, as outlined here: http://docs.valence.desire2learn.com/basic/fileupload.html#simple-uploads.
The file-size limit on the D2L Learning Environment side is set at deployment time, and generally not changed (without significant interaction with D2L Support and/or consultation with D2L around the client's deployment).
The file-size limit value is not a discoverable value to API clients, nor is it really intended to be. Generally, the limit is set to approximately 488 MB, but that's not a value you can assume: if you're working with a particular LMS back-end, you can seek to confirm the size with the admins for the LMS, but if you want to work with D2L Learning Service back-ends more generally, then you should assume that anything larger than 488 MB may cause you issues.
Note that the size limit applies to the entire payload in a simple upload packet: both the ones containing only file data, and the ones containing a JSON part followed by file data.