When it comes to checking the size of the file before it has been uploaded I have read in some places that this is possible but it is very much an advanced custom implementation and is not supported within Digital File Pro. The way that the Validation Toolkit and Digital File Pro work means that you will not be able to check the size or type of the file until it has been uploaded to the server.
The bindings for the upload should be labeled WA Upload Files (field name). In here are all of the bindings for the uploaded file including the error message, dubug info and the others. It is this error message that you want to place on the page. This should be on the page that has the upload server behavior applied to it.