This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 晊 is uncertain. It visually combines 日 (sun) and 至, but its original meaning and component function are not clearly established.
With 日 (sun) beside 至 (arrive), imagine the sun arriving at its peak, suggesting something large or great, which aligns with the kun-reading おお.きい (big).
For シツ, picture a sheet (シーツ) spread out under the big sun: sheet -> シツ, and the sun makes everything look large.