Use a Content Editor WebPart instead with the following HTML:

<iframe name="Iframe" src="yourURL" scrolling="no"
FRAMEBORDER="0" style="width:100%; border:0;  height:100%; overflow:hidden;">
</iframe>


1. Add a Content Editor WebPart to the Page
2. Click “Edit WebPart”
3. In Ribbon Tab “Format Text” click “Edit Source”
4. Copy and paste the code above into the “Edit Source”-window and replace “yourURL” with the URL you want to be displayed in the WebPart. Set the size of the WebPart to your needs.