As seen in the illustration above, the second candle completely overwhelms the prior candle. For a pattern to qualify as bullish engulfing, the high of the second candle should hit higher prices than the high of the prior candle. The same scenario applies for the low.
Ideally, the closing price (top of the body) should also be higher than the highest point of the wick of the prior candle. This scenario gives further significance to the second candle and shows that the bulls have control over the price action now.
The bearish candlestick pattern follows the same line of thought, the only difference is that it is a bearish reversal pattern that occurs at the top of an uptrend. The first candle is a bullish candle that signals the continuation of the uptrend, before the appearance of the powerful bearish candle that completely shuts down the prior candle.
Moreover, if the second candle is huge and long, it can practically close the door for you to open a trade, as your stop would be placed far away from the entry price i.e. high risk and not such high reward.
The best way to learn the strengths and weaknesses of the bullish and bearish engulfing patterns, as well as other candlestick formations, is to use the MetaTrader 5 trading platform and pay close attention to when these formations are created and how the price action behaves.