"Los sindicatos son grandes."
Translation:The unions are large.
The dictionary hints for grandes have been artificially pluralized, it appears. And while I am at it, syndicates was offered as a choice for the second word in the dictionary hint.
Adjectives (like grande) can have subtly different meaning depending on if they come before or after the noun. Ones that follow the noun usually describe an objective characteristic while ones that preceed describe subjective ones. For example, "el hombre grande" means "the large man" while "el gran hombre" means "the great man".
Check out this article: http://spanish.about.com/od/adjectives/a/adjectives_word_order.htm