According to the pokedex, Wailord is 14.5m tall.
If we scale him similarly to a blue whale, he would have a length of around 140m long. Since Wailord looks roughly cylindrical shaped, we can use the volume of a cylinder to find that he is 23,118 m3. Wailord's pokedex weight is 398kg. This means that Wailord's density is 0.017kg/m3 . To put that in perspective, water's density is 1000kg/m3 . At sea level and at 15 °C, air has a density of approximately 1.225 kg/m3