On the "Revenue" side you will quite probably need to have enough eyeballs that advertisers come to you directly to display ads and do so in volume.
On the "Costs" side you'd want to be big enough that you can just store your content in ~3 of your own datacentres, cache the "hot content" in a site or two per country then give away caches to ISPs (who will gladly host them in their own network for free).
Biggest cost will be bandwidth/streaming servers. Encoding/storage is comparatively cheap. If you were small you would likely start to do this from a few 100Gbps dedicated servers per continent. https://www.fdcservers.net/configurator?fixedFilter=15&fixed... If we set an average of 3-4Mbps per stream you're looking at each server handling 20,000 videos served and the hourly cost of the server would be around say $4/hour so you're looking at around $0.20 per 1000 video hours in theory, in practice it will be higher. Worst case closer to $0.50 per 1000 video hours due to utilisation rates.