IPhone : How to rescale text size?

I've created a modal on my app which the content is designed to fit onto the Iphone 6. However when I open the modal on the Iphone 5s, the content text is too big. Is there a way in the CSS to make the app resize the text based on the phone its on?

Currently I have in the CSS:

.firstParagraph {
  float: right;
  margin-left: 80px;
  font-size: 12;
}