var width = 0;
var length = 0;
document.getElementsByTagName("button").onclick = function(){
  width = document.getElementById("width").value;
  length = document.getElementById("length").value;
}
console.log(width, length)

<div class="setting">
        <p>Width: <input id="width" type="number"> px</p>
        <p>Length: <input id="length" type="number"> px</p>
        <button type="submit">Submit</button>
    </div>