My anemones always have kind of a ritual they do before they split. About once a week for a few weeks they shrink up real small and look withered and sick for a few hours, then go back to normal. After a few weeks(depends on feeding and happiness), they tear themselves in half. Then there are 2 anemones.
Mine usually get very large in size before they split. About 10" or more sometimes.
This is not to say that because your anemone looks sick that it is splitting. You should always watch it's behavior to see if it improves, and what changes happen.
This is before.