As the title says, usually up to 20Mb for an MP4 for example, the recording time for that size is usually enough to show examples that help support the ticket.
I have tried to upload a video to the feedback tracker that was about 3mins long, and it was 4mb ish in size. I have removed it as I uploaded to a different website and included the link, I personally think it makes sense to allow just MP4 uploads to be a maximum of 20Mb in size. Every other file format stays at the current upload limit.