The power of a Demon can be very tricky to measure. 

Size has no bearing on ability. Although imposing, larger demons are often weaker than smaller ones. Demons can "compress" their physical form through magical energy expenditure to create denser and more difficult to damage bodies.

Form has no bearing on ability. Complexity or simplicity or even the intimidating aspect of a Demonic form seems based more on personality than power.

Even the expenditure of energy is not accurate. Demons vary wildly in their ability to efficiently control their energy. A Demon that has no visible energy use is more likely to have a tight control on their energy. One that has constantly visibly energy use is likely very "loose" in their energy control. 

A glow around the body, energy wings, glowing eyes, glowing spikes, and any form of visible energy when they are not actively fighting is a sign that the demon is "loose" with their energy use.

However, a Demons with deep energy pools can afford to be "loose" with their energy use, and Demons with smaller energy pools must control that usage tightly.

As usual, the best solution is to strike with overwhelming force from a hidden location, regardless of the physical or energy form.

-Excerpt from the Slayers guide to Demonic Slaying, Revised Edition