Energy consumption would be limited by supply and energy dissipation, and energy dissipation (roughly speaking) depends on surface area and thermal conductivity. So, yes, for all devices made with a given shape and given materials, there is such an equation.

In theory, I guess, there’s also such an equation for the “upper limit” of heat dissipation for size, for a computer using the optimal shape and optimal materials. But since that probably concerns several-atom-thick devices made of diamond or something, that’s probably not practically useful anytime soon. :)