I have given permission for another artist to record a song that I have written and that song will also have a video which will most likely go on youtube. What are a songwriter's rights in terms of youtube revenue no matter how small ?
When you license your IP (like a song) you can specify the terms and conditions of its use by the licensee, including revenue shares from any derived work.
However, if, as your comment suggests, you grant an "informal" license, and later decide that you want to "firm things up" with a license having different terms, that's a matter you would have to either negotiate or litigate with your counterparty.
If you want a common reference point for negotiation of this sort of license, you might have a look at compulsory license terms.