As a rough guide I did this:
1) set the canvas to be 640x832 (iphone 4 4s size with address bar gone)
2) anchor the outer ui ellements to the edge. Note, there are plenty of bugs with anchor, especially if you want to center or use a different origin point, so I ended up doing my own anchoring by re positioning all the UI elements each time a window size change is detected (or you can do it every tick if you want).
3) I calculate if the main center game area will fit. If its too big or too small, then I just scale that one layer.
4) I use Crop, with a large backgroud image.
5) I always switch off webGL, as it screws up text and other items.
6) set the the "use ios retina display" to iphone only, as the default app res looks good in an ipad without it and no scaling needed.
Its pretty hairy.