Mammoth Uncensored Word List:
25 - 18 letter words starting with se

sedimentologically semibiographically semiconservatively semidomestications semimetaphorically semipermeabilities semiprofessionally semiquantitatively semirespectability sentimentalisation sentimentalization septemvigintillion septendecilliardth septendecillionths septingentillionth septuagintilliards septuagintillionth sesquicentennially sesquipedaliophobe sesquipedalophobes sesquipedalophobia sesquipedalophobic sestrigintillionth sesvigintillionths sexagintilliardths