It would scale to your screen, and tapping anywhere on the message body would take you to an app store. The 'close' button would be a fixed 10px square, grey on a black background. It would take at least 2 seconds to load in, and would jump around several times during that process with the button already active.

And once you manage to close out of that/click on it and back out of the app store there will be a small ovular request to open in the mobile app with an even tinier Close button.