Okay, so I have been able to find a definitive answer!
Naruto Narutimate Hero 3: Finally a Clash! Jonin VS Genin!! Indescriminate Grand Melee Tournament Meeting!!
is the longest titled anime with a large 107 characters or 87 characters without non-alphanumeric characters
Second Place goes to:
Digimon Adventure 02: Zenpen Digimon Hurricane Jouriku!! - Kouhen Chouzetsu Shinka!! Ougon no Digimental
With 104 characters, 85 alphanumeric
And Third,
Dragon Ball Z Special: Tatta Hitori no Saishuu Kessen - Freezer ni Idonda Z Sens hi Son Gokuu no Chichi
102 characters, 82 without.
I discovered this by writing a small Python script to iterate through a dump of AniDB's database of anime titles and check each one, which you can find on their site
If anyone is interested, you can find the script here: https://gist.github.com/ianfhunter/43599d30384b9b7aa0c6 (but I only hacked it up in 5 minutes, so it's nothing special)
(The shortest is C)