Spanish is one of the richest languages in vocabulary in the world. Also, it has a good number of long words. The RAE (Real Academia de la Lengua Española/Spanish Royal Academy) has determined a list of the Spanish longest words; here you will find six of longest and well-known Spanish words :
1. Esternocleidomastoideo (22 letters)
Meaning: Sternocleidomastoid, muscle of the neck.
2. Interdisciplinariedad (21 letters)
Meaning:Interdisciplinary, combining or involving two or more academic disciplines or fields of study.
3. Internacionalización (20 letters):
Meaning: Internationalization, to make something international.
4. Desvergonzadamente (18 letters)
Meaning: Shamelessly, lacking any sense of shame.
5. Desconsoladamente (17 letters)
Meaning: Inconsolably, that cannot be comforted
6. Electrodoméstico (16 letters)
Meaning: Home Appliance, electrical or mechanical machines which accomplish some household functions
Paralelepípedo (14 letters)
Meaning: Parallelepiped, a three-dimensional figure formed by six parallelograms
Caleidoscopio: (13 letters)
Meaning: Kaleidoscope, a toy consisting of a tube containing mirrors and pieces of colored glass or paper, whose reflections produce changing patterns.
As you can see, most of them are compound words (adverbs or two based-words). It could be a good exercise you try to practice their pronunciation!