All Lettuce

Harvest:
Loose-headed leaf lettuce when the leaves are large enough to use—just 2 to 3 inches long.

Bibb and Butterhead types of lettuce—with large ruffly outer leaves surrounding a soft, folded heart—when the leaves begin to cup inward to form a loose head or wait until they form a rosette at full size—6 to 8 inches across.

Romaine lettuce, also called Cos lettuce, when the leaves have elongated, formed midribs, and overlapped to form a fairly tight head–about 6 to 8 inches tall.

Harvest Crisphead lettuce—such as Iceberg with a solid, tight, spherical head–when heads are medium-size (6 to 8 inches), round, and firm.

For more information on growing Lettuce check-out:
https://harvesttotable.com/how_to_grow_lettuce/