Hi Phil – as @nathanael and @renaud can attest to, I notified the Prismic team many months ago about this issue on our team’s platinum plan – it exists when using the gatsby-source-prismic-graphql plugin as officially recommended (it does the same as you are, replacing the URL with the preview context inlined).
We have been waiting several months for an update to this issue or a solution, and in the meantime have resorted to creating alternate templates just to hold unique permutations of slices to work around the query length limit. Thankfully most of our pages are compact in length. I’m not sure what we will/would do in the future when we have a super long template with many slices required to be included.
This is also not limited to a preview context, it is a limitation to their graphql API generally. If you have too many slices on a given template type, the request will fail due to data length issues. When I first brought this to the team’s attention, @nathanael and @renaud said they would work on a way to query all slices for a given template in a more compact way, but that also has yet to come to fruition.
This is by far our biggest blocker/issue with the Prismic platform and has been for many months.