Pneumonoultramicroscopicsilicovolcanoconiosis contains 45 letters and to my knowlegde the longest word in the English Oxford dictionary. Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch is the name of the Welsh train station you can count the letters yourself. It is lots of Welsh words together rather than one word though.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.