What about canvas 2D? It has API for drawing an measuring text. You could use that. Set same font, font-size and you should be able to measure width of string from text box.
$('body').append('');
var canvas = document.getElementById('textCanvas');
var ctx = canvas.getContext('2d');
ctx.measureText(text);
var width = mes.width;
I used this kind of solution to acquire width of some text to draw it in WebGL. I worked just fine but I never tested this what going to happen when text is to big for canvas context, thought i think it will be fine because it is API for measuring text. But ho knows you should totally check it out! :)
