Thanks, guys. Actually I have width: 100% specified, but Ghost overwrites this attribute on the AMP page to a fixed one. Which won’t be a problem, but I can’t find the way to modify it.
I’ve tried to modify the template also (to give an exact width and height in the style amp-custom tag), but that doesn’t work.
Are you trying to change it to be smaller? Might be worth checking out the official AMP documentation, there’s some info here about iframe resizing Documentation: <amp-iframe> - amp.dev