Paste JSON and read it as a tree instead of as a wall of text. Branches collapse, so a large document becomes something you can actually navigate.
Paste JSON on the left. It is parsed on this page and never uploaded.
The problem with a big JSON file in an editor is that structure and content compete for the same screen. A tree separates them: you see the shape first and open only the branch you care about.
Array lengths and object key counts are shown on the collapsed node, so you can tell how big something is without expanding it.
Documents of a few megabytes render comfortably. Past that the browser rather than the tool becomes the limit, since every node is a real element on the page.