Finally I could guess, inspecting with Firebug, that another input tag was being placed in the DOM. So, the solution is modifying the new input style adding the new 'width' and 'height'.
it cost me many hours to discover.
I hope this can help anyone else.
Here is the code that Firebug shows:
<input id="file_upload" class="uploadifive-button" name="file_upload" type="file" style="display: none;"/>