I have often read words I didn't know, then I looked them up and knew them. If it is the word that best describes what you want to say then use it. A word is only too big if it is used to show off what big words you know or if it is out of place.
But be aware that for every reader who values a new word several more will just skip over it without bothering to look it up. Better still if the definition is inherant in the writing. Eg Twelve foals later she was sway-backed and nearly tailess, just a yaud that seemed out of place in a first class stable.