Transfer of thermal energy — practice question

There are four cubic copper vessels containing water. Every container has black-painted surfaces. Two of the vessels have side length $20\,\text{cm}$, while the other two have side length $40\,\text{cm}$. Water in two of the vessels is at $80\,^{\circ}\text{C}$ and water in the other two is at $30\,^{\circ}\text{C}$. Which vessel emits energy at the smallest rate?