It is slightly counterintuitive, but what is generally meant by a short candle is that the body (and not the entire candle) is shorter than usual when placed into the context of recent price action.

The following formula checks for the body to be more than 50% shorter than the mean body size over the most recent 15 candles.

ABS(O - C) < AVG(ABS(O - C), 15) / 2

There is no universally accepted definition of short, but most of the candlestick formulas I write will be using this formula as the definition.