We can use something called Kleiber's law (which I originally found here). It states that a power law describes the relationship between metabolic rate $q$ and mass of an animal $M$ (in kilograms):
You can then use this to determine the number of calories an animal consumes over the course of 24 hours, if we make certain assumptions about the constant of proportionality. As graphs from here show, it seems to fit animals of all different masses:
Kleiber's law isn't 100% accepted, I'll admit. The value of the exponent was debated for quite some time, and the relation doesn't hold universally (as noted in the page with the graph, plants don't follow the law). However, it's a pretty good basic estimate of a creature's caloric intake.