You can just change those "width" and "height" parameters to values you want:
<embed width="191" height="207" name="plugin" src="http://localhost:54149/Documento/VersaoView?chave=FDC4875EE17FB17B" type="application/pdf">
width="191"
tells browser to set width to 191 pixels.
This method loads the whole pdf though, not just a thumbnail.
For automatic thumbnails generation, you might use PHP as described here
How do I convert a PDF document to a preview image in PHP?
Or you could just create a thumbnail image manually, and then add it on your website as an image with link to the pdf file:
<a href="http://example.com/mydocument.pdf">
<img src="http://example.com/mydocument-thumbnail.jpg" />
</a>