The word a as pronounced by a North Carolinian with true North Carolinian accent. Now, some would call that 'a' a diphthong, but I hear two most decidedly distinct syllables, especially when you hear bonafide two-syllable words with the final one clipped off altogether.

aa, a type of lava, is pronounced 'ah ah', according to our tsuwm, so that's a two-syllable word of two letters.

Don't have any time now to consider long one-syllable words, but 'straight' comes to mind right away. And that's 8 letters.

Have fun with this thread!