What is Chopin's shortest piece? Is it the Minute Waltz?
Not by a long shot; a number of the preludes hover around the 30-second mark or even below if played quickly (I think the E-flat minor prelude may just about be the shortest by a few seconds), and more still are shorter than 60 seconds. In terms of number of bars, well, the E major prelude is only 12 bars long, but it's marked Largo so it takes about a minute to play. Same for the C minor prelude, which is only 13 bars long.
The grand champion of short preludes in the 24 major and minor keys, though, is probably Hummel, whose 24 Preludes Op.67 take a total of eight minutes to perform and vary between 10 seconds and 30 seconds in length. I suspect they're not intended as stand-alone pieces, though.