Actually, in principle the correct number of letters in the longest word is 1 divided by 0, i.e. infinity. However if our corpus is the sum of all texts ever written and all words ever spoken and since human existence is finite my guess the number of letters is at least 100. Here’s a funny one, but please don’t ask me to translate it.
Donaudampfschifffahrtskapitänswitwenversicherungspolicenvermittlerprovisionsvorsteuerabzugsberechtigungsklausel