The smallest "resizable" implementation there is

resizable.css

.resizable {
    resize: both;   /* Options: horizontal, vertical, both */
    overflow: auto; /* Options: scroll, hidden, auto */
}

resizable.js

var registerResizeHandler = function (element, handler) {
    var width = element.style.width,
        height = element.style.height;

    element.onmouseup = function () {
        if (width !== element.style.width || height !== element.style.height) {
            width = element.style.width;
            height = element.style.height;
            handler(width, height);
        }
    };
};