This CodePen by Chris Coyier will let you use sliders to dynamically change the font size on a page
http://codepen.io/chriscoyier/pen/tvheK
This demo goes with one of the readings on Fluid Measurements, but I am including it here again just in case you missed the code. You can read it here if you want to know how to use the tool. https://css-tricks.com/rems-ems/