If you want to desing the app in PaintCode, the best approach is probably using Frames. By setting resizing behavior you can get the best layout on all iPhone screen sizes. You can then use one canvas as main and have additional canvases with Symbol for other sizes. See attached image.

Keep in mind that when designing whole screen in PaintCode, it then cannot be used directly from code. However, it allows you to break the design into bars, buttons, icons, and other controls, which can be used both in you composed design and from the code (using StyleKit).
Some people even use the PaintCode to produce App Store screenshots, instead of snapshotting the running app.
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