Ok, after an heavy trial-and-error session, I found the solution.
I was using the wrong value from the binding panel.
Now I used the "file size" that you find below the form field (not the one under "upload file (my_field)").
I set a limit using the international format and set it to 5120000 to limit file size to 5MB.
Now it works.
Thanks to all for the help.
Just a note of disappointment for the online help system. I can't find any reference to all the bindings value created by the WA extensions related to file upload.