There are 16 13-letter words that start with 'is'.
Note: These 16 13-letter words that start with 'is' are found from the USA & Canada version of the tournament Scrabble word list.
Also see words start with iso, isa, isl, iss, isc, ist, ise, isb, isi, ism.
3-letter words start with 'is' (1)