Recommended total file size for lower end phones

This is a question for those of you who create material for lower resource settings -

Most of our users have poor internet and lower-end, memory-limited phones. Apart from our books for children with low vision which means we need high quality images, we also do sign language books, which means video files in the book.

Is there a page size or book size which I should set as a wise limit? I know that’s a very hard question, but is there a ballpark for a lower end/ limited type phone?