About 50 cm including tail

Photo from Wikimedia Commons
Platypus
The venomous, egg-laying, duck-billed mammal that broke taxonomy/Small Animals
The platypus (Ornithorhynchus anatinus) is one of only five species of egg-laying mammals. It has a duck-like bill, a beaver-like tail, venomous spurs on its hind legs, and electroreceptors in its bill that detect the electric fields of prey. When European scientists first examined a platypus specimen in 1799, they thought it was a hoax — someone sewing bits of different animals together. It is arguably the most improbable animal on Earth.
Measurements
About 2.4 kg for males; females are smaller
About 3.6 km/h in water
About 6 cm; packed with electroreceptors
15 mm keratinous spur on hind ankles
Flat, beaver-like tail for fat storage
About 10 days; one of the shortest for any egg-layer