My 'standard' is 59 inches because that is what I prefer to play with, so I design around that. My butt and joint diameters are on the high side as well, but that is beside the point.
I have not made anything longer (I'm just on my second cue!) but I don't think the lathe I have access to would allow much longer- maybe 62 inches overall, but not longer than that with any comfort.
And, I agree 100% that you have to define your wants and needs before you seek a maker. (When I made custom guitars I used to get calls/visits all the time from players who wanted a custom guitar and when I asked them exactly what they wanted, they had no idea!) If you HAVE to have a 64" long cue, then you find a maker comfortable with making the cue. I mean, you may find someone physically able to make the cue, but it may not have the 'hit' you want in the cue...
Something I have seen recently is a center section between the butt and shaft that extends a standard cue by how ever many inches you need. If you have a cue you like, and wanted to try that idea, I'd be happy to try and work with you on that. I'm sure others can as well... Food for thought!