The dictionary form is "맵다." This is called a "ㅂ irregular" in which the ㅂ gets replaced by 우 when a vowel is added to the stem (맵). So you start with 맵다 from which you get the stem "맵." You conjugate it to "맵은" like you said, but since a vowel is added it changes to "매운."
The basic form of "to be spicy" in Korean is 맵다. This makes it a ㅂ irregular which is conjugated as such : 매워요, 매운... A good example is 새롭다 which becomes 새로워요, 새로운... Go to dongsa.net for conuugations.
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You add ㄴ at the end of a verb stem if it ends in a vowel, and 은 if it ends in a consonant. Therefore, 매우 + ㄴ = 매운 (spicy).
작 (verb stem meaning "short") end in a consonant, so
작 + 은 = 작은 (short)