The current preview screen for email blasts makes truly previewing an email quite hard, because it's width is not representative of any modern viewport and the height is constrained in much the same manner.
That said, simply changing `/preview` in the URL to `/html_preview` (the link being <iframe>'d into the preview screen) reveals the email in full-width.
It's easy to make this change to the url, but it's not obvious that it exists in the first place despite being invaluable while editing a theme, for instance. At risk of being too prescriptive, an easy solution would be to hyperlink the current "HTML" title in the preview window to the `/html_preview` variant, set to open in a new tab, so that users can quickly and easily access a full-screen preview of their blast.
This isn't something we can have a timeline on now but we will keep it in mind - I'll also suggest to the writing team that they add this tip to the docs.