Snapmirror destination will always be the exact replica of the source, vol options like autogrow, snap auto delete, vol guarantee etc.
So even if autosize is turned on in the destination, on snapmirroring it will follow the source(autosize is off on source), only on mirror break the autosize will be turned on.
But since fs_size_fixed set to ON in the destination even on mirror break, autosize cant take effect.
so you will have to turn fs_size_fixed to off , so autosize can take effect.
A better way would be to create a none guaranteed volume with 2x size than your source so the volume doesn't consume any space from the aggr.
On snapmirroring the destination volume size will the exact size of the source volume and only that much space would be consumed from the aggr.
Even if the source grows since the snapmirror destinatoin volume actual size is 2x the source it will grow till that size, with out snapmirror failing.
Even better would be to create aggr sized volume of none guarantee.
Regards
adai