I have some tall lettuce plants (about 4-5 feet tall). I have no idea what type this is or how or when to harvest them.
Lettuce varieties very and so do there growing patterns, but generally would stay much smaller.
The back of your seed packets contain plant size information.
It may be that your plants have bolted. This can be due to weather conditions, particularly warm temperatures in the evenings can cause your plants to bolt.
Here are some articles that will help you.