That's one of the main differences between English and French: in French, adjectives (small, nice, cheap...) agree in gender and number with the thing or person they refer to. Therefore, they will be either masculine or feminine, and either singular or plural, just like nouns.
Let's see how you pluralise simple adjectives:
Case of mixed gender:
and the more advanced: Adjectives ending in -s or -x only change their feminine forms when plural
Examples and resources
"Plurals in French" by www.frenchspanishonline.com