Determining image file size + dimensions via Javascript?

Edit:

To get the current in-browser pixel size of a DOM element (in your case IMG elements) excluding the border and margin, you can use the clientWidth and clientHeight properties.

var img = document.getElementById('imageId'); 

var width = img.clientWidth;
var height = img.clientHeight;

Now to get the file size, now I can only think about the fileSize property that Internet Explorer exposes for document and IMG elements...

Edit 2: Something comes to my mind...

To get the size of a file hosted on the server, you could simply make an HEAD HTTP Request using Ajax. This kind of request is used to obtain metainformation about the url implied by the request without transferring any content of it in the response.

At the end of the HTTP Request, we have access to the response HTTP Headers, including the Content-Length which represents the size of the file in bytes.

A basic example using raw XHR:

var xhr = new XMLHttpRequest();
xhr.open('HEAD', 'img/test.jpg', true);
xhr.onreadystatechange = function(){
  if ( xhr.readyState == 4 ) {
    if ( xhr.status == 200 ) {
      alert('Size in bytes: ' + xhr.getResponseHeader('Content-Length'));
    } else {
      alert('ERROR');
    }
  }
};
xhr.send(null);

Note: Keep in mind that when you do Ajax requests, you are restricted by the Same origin policy, which allows you to make requests only within the same domain.

Check a working proof of concept here.

Edit 3:

1.) About the Content-Length, I think that a size mismatch could happen for example if the server response is gzipped, you can do some tests to see if this happens on your server.

2.) For get the original dimensions of a image, you could create an IMG element programmatically, for example:

var img = document.createElement('img');

img.onload = function () { alert(img.width + ' x ' + img.height); };

img.src='http://sstatic.net/so/img/logo.png';

How about this:

var imageUrl = 'https://cdn.sstatic.net/Sites/stackoverflow/img/sprites.svg';
var blob = null;
var xhr = new XMLHttpRequest(); 
xhr.open('GET', imageUrl, true); 
xhr.responseType = 'blob';
xhr.onload = function() 
{
    blob = xhr.response;
    console.log(blob, blob.size);
}
xhr.send();

http://qnimate.com/javascript-create-file-object-from-url/

due to Same Origin Policy, only work under same origin

Check the uploaded image size using Javascript

<script type="text/javascript">
    function check(){
      var imgpath=document.getElementById('imgfile');
      if (!imgpath.value==""){
        var img=imgpath.files[0].size;
        var imgsize=img/1024; 
        alert(imgsize);
      }
    }
</script>

Html code

<form method="post" enctype="multipart/form-data" onsubmit="return check();">
<input type="file" name="imgfile" id="imgfile"><br><input type="submit">
</form>

Getting the Original Dimensions of the Image

If you need to get the original image dimensions (not in the browser context), clientWidth and clientHeight properties do not work since they return incorrect values if the image is stretched/shrunk via css.

To get original image dimensions, use naturalHeight and naturalWidth properties.

var img = document.getElementById('imageId'); 

var width = img.naturalWidth;
var height = img.naturalHeight;

p.s. This does not answer the original question as the accepted answer does the job. This, instead, serves like addition to it.