I’m developing a custom theme locally with Shopify CLI.
I’ve added a video to a page (video is in assets folder, is 230MB file size, and 13 minutes long - it’s not a product vide - its educational & not on a product page)
You can try with a few incremental file-sizes to see where the upload fails. (you can also note the % upload after which the server returns the error to get an approximate upload length limit)