d3-text-measure.js

function textSize(text) {
    if (!d3) return;
    var container = d3.select('body').append('svg');
    container.append('text').attr('x', -99999).attr('y',-99999).text(text);
    var size = container.node().getBBox();
    container.remove();
    return { width: size.width, height: size.height };
}

// Usage: textSize("This is a very long text"); 
// => Return: Object {width: 140, height: 15.453125}