How small is too small?
A: Sika have a reputation among stalkers for being tough and when shot they are frequent runners.
They are certainly larger than roe but critically when compared as a quarry species, they are more alert, aware and cautious as they go about their daily routine and movement patterns. Often they don’t emerge until last light and are then very tentative. This may help to explain how they got their reputation because it’s true that an alert and aware animal will react differently to a settled one when shot. This is particularly relevant if the shot placement does not result in an instantly paralysing collapse.
As to a larger calibre, that’s a question only you can answer. When it comes to shooting wild deer the .243 calibre rifle – with its long-established tried and tested reputation and ready supply of factory ammunition – will handle humanely anything you might be likely to encounter in the UK. The only possible exception to this might be the very large red stags of the sort found in some parts of England.
From experience with sika, I have found that the .243 has proved to be up to the job.