To get the errors to display on the page you will need to go to the Digital File Pro bindings and insert the 'Error Message' binding onto the page where you would like to see the error. It will let you know what errors occurred with the upload of the file.
When you were able to upload the file that was too large how did you have your upload server behavior setup for this? It should definitely restrict the file based on the size or type if you specified these options. If you have set the options like this please post back with a copy of your upload page and include a link where this can be tested out.