EjemploEdit

HTML

<!-- Learn about this code on MDN: https://developer.mozilla.org/es/docs/Web/API/FileReader/readAsDataURL -->

<input type="file" onchange="previewFile()"><br>
<img src="" style="heigth: 200px; width: 200px;" alt="Image preview...">

JavaScript

function previewFile() {
  var preview = document.querySelector('img');
  var file    = document.querySelector('input[type=file]').files[0];
  var reader  = new FileReader();

 reader.onloadend = function () {
   console.log(preview.width.toFixed(0))
   console.log(preview.height.toFixed(0))
   if (preview.width.toFixed(0) > 400 || preview.height.toFixed(0) > 400) { 
     alert('enter a 400x400px or less image');
     return false;
   } else {
      preview.src = reader.result;
   }
 }
 if (file) {
   reader.readAsDataURL(file);
 } else {
   preview.src = "";
 }  
}