This is the source file with both issues fixed. I've commented where code has been added or edited.
The issue with the width and height was fixed in the way I already said.
The issue with the insets seems to be caused by the fact that MigLayoutFX was not calculating and setting its sizes.
I've fixed that by setting the sizes of the MigLayoutFX Node when the grid is created (createMigLayoutGrid()). Also I've overriden the computeMin/Max/PrefWidth/Height methods of the Region class, in order the size of the MigLayoutFX Node to be recalculated when resizing.
I found about this issue when I was creating a MigLayoutFX Node with several components in it and then adding it to another layout (see Example.java). When using the fixed version of MigLayoutFX2, everything works fine (see screenshot).
Windows XP SP3 x86, JDK7, JavaFX b38.