He was originally around 75 cm but was shortened to around 60 cm, which makes him an Oowakizashi (大脇差). On that part that was shortened was inlayed his previous owner's name. But now what's only left was "Hashiba Gorou Zaemon no Jounaga" (羽柴五郎座衛門尉長) which was the previous self-chosen name of Toyotomi Hideyoshi when he was a retainer of Oda Nobunaga.
Regarding his original size and his claim of formerly being an ootachi, apparently back when he was forged, that size was already long enough to be called ootachi, though in present times it's only qualified as a tachi.
The artist confirmed that the cape-like cloth hanging on his shoulder is a shiroshouzoku (白装束), white clothing often worn by Shinto priests and miko. Additionally, it's also a clothing people put on the dead. It can be assumed that it's related to the story of him cutting down both the ghost child and woman.
Nikkari's name comes from a tale in the book "Kyouhou Meibutsu Chou" (享保名物帳 - Kyoubou's Book of Finely Crafted Swords). Tales of a monster in the mountains of Hachiman (八幡) caused Lord Nakajima Shuuridayuu (中島修理太夫) to grab the sharpest Aoe Ootachi he owned, and go to the mountain to slay the creature. There, he met the ghosts of an infant and a woman, both of which tried to harm him. In defense, he slashed both with the ootachi, and the ghosts vanish into thin air. Next day, he went to check the bodies, but instead found two stone lanterns cut in half. Seeing this as a success, he decided to name the ootachi "Nikkari Aoe", with Nikkari being a reference to the eerie/creepy grin of the ghost woman.