In order to do this, you would need to fix the user's viewport so that they cannot pinch zoom the page, and then use a touch events library like Hammer.js to attach a callback to the pinch zoom gesture that appropriately resizes the element on the page you'd like to scale.

viewport fixing happens in the head element of your html:

<meta name=viewport content=width=device-width, maximum-scale=1.0>

you would use hammer.js to detect a pinch zoom gesture, and the library gives you a very detailed event.gesture object which you can use to detect how much/how fast the user is zooming.

The change in distance between the 2 touch points while pinching is represented by event.gesture.scale (see hammer documentation), if the scale increases, increase the text accordingly ... if it decreases decrease the text-size. Use Math:

$('body').hammer().on(pinch, function(event) {

    console.log(event.gesture.scale);

    // do math...

});

I imagine you get the idea...