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
As of Couchbase 4.5 you have to modify the app.min.js file not the documents.js. To permanently fix try:
sed -i 's|return getStringBytesFilter(json)>docBytesLimit;|return false;|' /opt/couchbase/lib/ns_server/erlang/lib/ns_server/priv/public/ui/app.min.js
Couchbase console is here to help you, but when you deal with large data it is better to use a SDK and modify your document using some code. You can find all the SDKs here: http://www.couchbase.com/develop
Is is possible with your environment?