Currently, each part should have exactly the size that was returned from post-files-upload-sessions (Link: https://developer.box.com/reference/post-files-upload-sessions/) API call in Upload session object (Link: https://developer.box.com/reference/resources/upload-session/#param-part_size)

Depending on a user's network configuration, allowing a user to determine the specific part size for chunked uploads can allow them more control in completing these uploads quicker, and preventing failures based on network restrictions they may possibly have in place.