Check file size before upload

I am using this javascript that I got from here, and it works perfectly for what I need it to.

var _validFileExtensions = [".jpg", ".jpeg"]; 

function File_Validator(theForm){
    var arrInputs = theForm.getElementsByTagName("input"); 
    for (var i = 0; i < arrInputs.length; i++) { 
    var oInput = arrInputs[i]; 
    if (oInput.type == "file") { 
        var sFileName = oInput.value; 
        var blnValid = false; 
            for (var j = 0; j < _validFileExtensions.length; j++) { 
                var sCurExtension = _validFileExtensions[j]; 
                if (sFileName.substr(sFileName.length - sCurExtension.length, sCurExtension.length).toLowerCase() == sCurExtension.toLowerCase()) { 
                    blnValid = true; 
                    break; 
                    } 
                } 

                if (!blnValid) { 
                    alert("Invalid image file type!"); 
                    return false; 
                } 
        } 
    } 

return true; 
} 

Now, I was wondering if, in addition, I could check the file size and fail if the file is bigger than 500kb -> all before pressing the submit/upload button?

EDIT

After looking into what PHPMyCoder suggested, I end up solving by using this javascript code:

<script language='JavaScript'>
function checkFileSize(inputFile) {
var max =  3 * 512 * 512; // 786MB

if (inputFile.files && inputFile.files[0].size > max) {
    alert("File too large."); // Do your thing to handle the error.
    inputFile.value = null; // Clear the field.
   }
}
</script>

This checks the file size, and alert the user before submitting the form.