Back in the olden days of e-readers.
260K was the limit of the 'current section' that could be active. It (the setting)
is still present in the
Conversion Output options for EPUB in Calibre.
This does not make a separate book, just breaks up the internals so the load faster (the more common break these days is by Chapter).
As to Splitting a book simply by size??? How does that make sense? That would be similar to just Unzipping an EPUB and saving the debris
, that will soon get lost or out of order.