Las pizarras son largas
Pizarra is a fem. singular noun. Generally speaking, fem. singular nouns interact with their corresponding adjectives matching each other when it comes to their quantity and if they are masculine or femenine. So: pizarra larga (singular), pizarras largas (plural); coche azul (singular, masculine), coches azules (plural). Note you add an -es termination when the word does not end with a vowel (azul, azules; lápiz, lápices, and so on. Such rule applies to both nouns and adjectives).