Run javascript function on load

window.onload = changeFontSize;
window.onresize = changeFontSize;

alternatively, you can use

window.addEventListener("onload", changeFontSize, false);
window.addEventListener("onresize", changeFontSize, false);

  var text = document.getElementById("myText");

  function changeFontSize() {
    if (window.innerWidth < 600) {
      text.style.fontSize = "70px";
    } else {
      text.style.fontSize = "100px";
    }
  }

  window.onresize = changeFontSize;

  setTimeout(changeFontSize, 3000);

<p id="myText">Test</p>

Basically you had a syntax error in:

setTimeout(function()  changeFontSize, 3000);

You don't need the function()

To start with, setTimeout() isn't used the way you wrote it, it should be setTimeout(changeFontSize, 3000); instead. That being said this would work:

<script>
    var text = document.getElementById("myText");

    function changeFontSize() {
        if (window.innerWidth < 600) {
            text.style.fontSize = "70px";
        } else {
            text.style.fontSize = "100px";
        }
    }

    window.onresize = changeFontSize;

    changeFontSize();
</script>

As was also said by jperelli, window.onload = changeFontSize would also work.

If you want absolutely use onresize instead onload (I suppose that you want to achieve something responsive), you need to wait the page is loaded, otherwise the object myText will not the object will not yet been initialized in the variable text ('cause it's not yet available in the DOM). Note that in this case the use of a setTimeout (you wrong the syntax anyway) is not necessary.

This works:

<script>

  function changeFontSize(){
      if (window.innerWidth < 600) {
          document.getElementById("myText").style.fontSize = "70px";
      } else {
          document.getElementById("myText").style.fontSize = "100px";
      }
  }

  window.onresize = changeFontSize;

</script>