While you are waiting answer from devs I can share my "knowledge".
There is no restriction on file size now. I had flawless syncronization with 50MB attachments on OneDrive, but I am sure it would be OK with GBs too.
The main problem with OneDrive is the size of note itself. Somehow it interferes with encryption. This bug can lead to creating a huge chunk of encrypted text in your note what is impossible to sync with OneDrive. This chink might be 2MB as it was in my issue. So you can experience problem with a small note without attachments and do not experience any problems with GBs attachments.
Another restriction is amount of files in one folder in OneDrive which is 150000. It might be achieved if you have many notes with many attachments.