again, it depends on the total volume of the 3 tanks above. your first drawing shows either a 15g or 30g. that will make a difference.
i usually go for 1/3 of the display volume, but 1/4 of the display volume will be fine.
the size will not only increase the volume of your water column (helping to stabilize your water parameters), but will ensure your sump does not overflow during backfill (when the pump is turned off)

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