Hello bg,
you can add some javascript which test the size on the onchange event of the upload_file field.
$('#fieldID').on("change", function() {
alert(this.files[0].size);
});
You can add that code in the script textarea of the configuration of the file, after clicking on the down orange arrow at the bottom of parameters.
Note that this will only work with modern browsers (IE>10).
Best regards.
Lionel