I tried to edit document via couchbase console, and caught this warning message:
Warning: Editing of document with size more than 2.5kb is not allowe
It seems that the UI code will actually make the request and get the document back, but then refuse to show it if it's too big. So, you can actually just open up the browser developer tools, find the request for the document you want in the network traffic, and the document itself is right there in the response. Copy-paste into a pretty-printer, and you're done.