As a general rule, does the heart rate of an animal increase or decrease with size? Is there a certain pattern?
The heart rate increases when the size of the animal decreases.
In general, the size of an animal's heart represents a constant fraction of its body size. This means that small animals have small hearts and, conversely, large animals have large hearts.
Design and cardiac function are closely related to the heart's need for oxygen. The rate of oxygen consumption, relative to body size, decreases with increasing size. It can be assumed that, in general, the beat volume is proportional to the size of the heart. Thus, the relative decrease in the need for oxygen and blood flow in large animals is accompanied by a decrease in heart rate.
This general rule can have exceptions since within each class of animals, the most active have relatively larger hearts and a higher frequency than the others, regardless of their size.