Final answer:
The Helmet Jellyfish belongs to the class Scyphozoa, which can grow up to 40 cm in length, with the largest species, Cyanea capillata, reaching up to 2 meters in diameter. The class Cubozoa, also known as box jellyfish, typically grows between 15 to 25 cm.
Step-by-step explanation:
The Helmet Jellyfish, also known as members of the class Scyphozoa, portrays a bell-like body shape and is among the marine jellies. The size of these creatures can vary significantly, ranging from a diminutive 2 cm to an impressive length of 40 cm.
However, when it comes to the largest known species within this class, the Cyanea capillata stands out, boasting a massive diameter that can reach up to 2 meters. In contrast, the class Cubozoa, which includes the box jellyfish, generally reaches sizes between 15 to 25 cm, yet they exhibit many of the same morphological and anatomical features as their Scyphozoan counterparts.