The problem is that aside form the fact that the users setting will override your font settings, you don't know definitely what fonts they have installed, you don't know what platform they are on and yo udo not know the rendering characteristics for their monitor.
You have to get the sizing information from the client. Here is the easy way using client side Javascript:
Put the text in a span with an id.
<span id="sizer">your text here</span>
Then just query the properties:
alert(document.getElementById('sizer').offsetWidth);
You have to wait for the body to load. If you need it on the user initial entry to the site. You could give them an index page with nothing on it but that with the text set the same color as teh background and then set the cookie with the value and re-direct to your real page. If they are going to be coming through another page you can capture from any of th eother pages and save it to the cookie.