Complex sentence: Breeding adults are 63 cm to 75 cm in length with a 100 cm to 122 cm wingspan, shaped like a smaller, sleeker version of the Great Northern Diver.
Split into simpler sentences: Breeding adults are 63 cm to 75 cm in length with a 100 cm to 122 cm wingspan. They are like a smaller, sleeker version of the Great Northern Diver.