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The best size is as small as you can possibly make them. The larger the files, the worse the performance will be, but also the epub reader will insert page breaks at awkward places if the file is too large.
It is best to start a new file at each natural page break, and I would avoid anything larger than about 120KB in any case. I think it is better to choose where to split the file yourself rather than let the epub reader insert a page break or Calibre split the file automatically in an unexpected place.
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