This library seems to provide options as getters and setters. Therefore, to print sendBufferSize, you must do the following.
Check out the code around line 80 https://github.com/JustinTulloss/zeromq.node/blob/master/zeromq.js
var sys = require("sys");
var zmq = require('zeromq');
s = zmq.createSocket('req');
console.log(s.sendBufferSize);
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