How can I get a file's upload size using simple Javascript?

See http://www.w3.org/TR/FileAPI/. It is supported by Firefox 3.6; I don't know about any other browsers.

Within the onchange event of a <input id="fileInput" type="file" /> simply:

var fi = document.getElementById('fileInput');
alert(fi.files[0].size); // maybe fileSize, I forget

You can also return the contents of the file as a string, and so forth. But again, this may only work with Firefox 3.6.

Now it is possible to get file size using pure JavaScript. Nearly all browser support FileReader, which you can use to read file size as well as you can show image without uploading file to server. link

Code:

    var oFile = document.getElementById("file-input").files[0]; // input box with type file;
    var img = document.getElementById("imgtag");
    var reader = new FileReader();
    reader.onload = function (e) {
            console.log(e.total); // file size 
            img.src =  e.target.result; // putting file in dom without server upload.

   };
   reader.readAsDataURL(oFile );

You can get file size directly from file object using following code.

 var fileSize = oFile.size;

Other that aquiring the filename there is no way for you to find out any other details about the file in javascript including its size.

Instead you should configure server-side script to block an oversized upload.