Yeah. Enlarging the Sir Kenny stand on its own would probably add 7-9k, but I'm not sure you can add much to The Kop as a single tier to make the work financially viable. It would probably get a facelift to bring it in line with the rest of the stadium though.
And of course going bigger brings up the issue of transport infrastructure in the area, which is already creaking on match days.
Using some back of a fag-packet calculations regarding safe-standing, and assuming the increased capacity could pass planning permission in regards to transport etc, and the gradient of the stands is in line with regulations, and it is possible to add in extra entry points to the Kop without big structural reform etc (a lot of 'if's' basically!).
If it was a 1:1.5 ratio you could add 6,000 to a safe-standing Kop, plus 3,000 to a safe-standing Lower Anny - which would see the capacity reach near 70,000 without having to completely rebuild any more stands. That would be for non-Champions League games at least (although who knows - if the Premier League implemented safe standing alongside the Bundesliga, that might be the start of Champions League regulations eventually come into line as well. They finally got around to relaxing alcohol in stadiums afterall).
That would be 27,000 standing places in Anfield - not far off the old Kop.