小さな・大きな (連体詞): small / big, only before a noun

小さな and 大きな are 連体詞 (れんたいし): words that only come right before a noun. They mean the same as 小さい and 大きい, which are the words to learn and count.

小さな町 = 小さい町 "a small town". But only 小さい can end a sentence or change form: 町が小さい "the town is small" (not 町が小さな), 小さくない, 小さかった. 大きな works the same way next to 大きい. 小さな and 大きな are not な-adjectives: there is no 小さだ.