There are two decent options I can think of: adjusting the font size and adjusting the scale of the text element. Very similar in either case.
Font size:
document.getElementById("textSize").addEventListener("change", function() {
var size = this.value;
text.fontSize(size);
layer.draw();
}, true);
Fiddle: http://jsfiddle.net/pDW34/8/
Scale:
document.getElementById("textSize").addEventListener("change", function() {
var scale = this.value/100*4;
text.scale({x: scale, y: scale});
layer.draw();
}, true);
Fiddle: http://jsfiddle.net/pDW34/9/