The original form of the adjective meaning very small is minuscule. It comes from the Latin minusculus, which in turn is a diminutive of minus, so using u in the second syllable of minuscule is unassailable from an etymological perspective. Still, miniscule, with an i instead of a u in the second syllable, has been so common for so long that it is now considered a variant of the original word. It is listed in dictionaries and even appears fairly often in edited writing. Some people still consider it incorrect, though, and the original form is undoubtedly the safer choice.