This is such a great idea, especially for those of us who build web sites. Its great to see the two views side-by-side, and the simplicity is fantastic. There are issues, though.
1. It needs some level of control over the width of the mobile pane (the scrollbar takes up space, throwing things off, and its crucial to be able to check different widths)
2. Being able to limit the height of the mobile pane would be nice too, so that one can check where the fold is
3. Width X Height is standard, but dimensions display as H X W
4. No tabs, boo
For the dimensions, hopefully theyll add some presets that match common screen widths (e.g. 320w, 1024w, etc.). Even better would be the ability to define your own size settings.
Good start, and I look forward to the next version.