will complain if any of the .html files in the ePub is 304 KB (=311296 bytes) or larger. But you'll only see the warning message if you choose Item Info... from the Reading menu.
BTW, you can have Sigil split large .html files for you. Simply insert <hr class="sigilChapterBreak" />
before the split points and then select Split at Chapter Markers from the Edit menu. (You'll have to re-generate the toc afterwards.)