Here is the (erm, one) thread about hexediting the vanilla (aka eMMC) image. But I forgot about that signing / aegis with N9, so might be undoable that way.
Furthermore when looking at that partition config there is no mentioning of p1 size, so probably it will partition from end (so my idea is futile).
But juiceme's idea is a quite nice one, too ...

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about some other questions:
you need to look at the eMMC image
what device do you have? 16G or 64G. First one will still be isable but user space for doc/music/vid restricted to only <4G.