<script type="text/javascript">
function AlertFilesize(){
if(window.ActiveXObject){
var fso = new ActiveXObject("Scripting.FileSystemObject");
var filepath = document.getElementById('fileInput').value;
var thefile = fso.getFile(filepath);
var sizeinbytes = thefile.size;
}else{
var sizeinbytes = document.getElementById('fileInput').files[0].size;
}
var fSExt = new Array('Bytes', 'KB', 'MB', 'GB');
fSize = sizeinbytes; i=0;while(fSize>900){fSize/=1024;i++;}
alert((Math.round(fSize*100)/100)+' '+fSExt[i]);
}
</script>
<input id="fileInput" type="file" onchange="AlertFilesize();" />
Работа над IE и FF