UPDATE 2013
as of this edit, the File API is supported in all major browsers, and in IE as of version 10
http://caniuse.com/#search=file%20api
You may still wish to use SWFUpload if you still need to support IE9 and under, though at this point it should probably be more of a fallback, since the html5 file api has support on mobile platforms where SWFUpload cannot reach. The html5 file api is based on firefox's file api as noted below.
See also this duplicate question
Client Checking file size using HTML5?
UPDATE: Firefox has changed their API to this https://developer.mozilla.org/en/DOM/FileReader
You can do it in firefox like so
html:
<form action="" method="get" accept-charset="utf-8">
<input type="file" name="file" value="" id="file">
<p><input type="submit" value="Continue →"></p>
</form>
javascript:
var filesize = document.forms[0].file.files[0].fileSize
if there's a way to do this in IE, I don't know it. It probably involves activeX or some other such rubbish.
edit:
I found this here, HOW TO DO THIS IN IE
<head>
<script>
function getSize()
{
var myFSO = new ActiveXObject("Scripting.FileSystemObject");
var filepath = document.upload.file.value;
var thefile = myFSO.getFile(filepath);
var size = thefile.size;
alert(size + " bytes");
}
</script>
</head>
<body>
<form name="upload">
<input type="file" name="file">
<input type="button" value="Size?" onClick="getSize();">
</form>
</body>
</html>
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