Currently the quickest way to maximize a pre-selected iframe is by using the keyboard shortcut: "Ctrl + F1". If the page has more than one iframe and you have yet to make a selection, you'll be prompted to select an iframe to maximize.
That been said, I'll definitely keep what you said in mind for the next release. I would also like to point out that this extension is open-source (https://github.com/haroun-b/iframe-resizer). So please feel free to fork it, if you need to make adjustments before then. You may also submit a feature request so someone else might work on it.
