Not only are you choosing its place within the natural (and unnatural) order of things, but you're effectively choosing the methods by which the players will dispatch of them. Choosing the right fit for this means having a clear concept, both of the creature's abilities as well as how they exist within their respective ecosystems. While the book provides ample examples, it's definitely more art than science.
The obvious first step is to compare your monster to one of those examples. Alternatively, ask yourself if it would win a fight against any of the the monsters at that Order of Might. If the answer is yes, go to the next level. If it could go either way, you're probably at the right number. If the answer is unequivocally no, you've gone too high.
Should nothing jump out at you, consider how supernatural this creature is. Generally, the more mundane stuff (for a fantasy world) falls at 4 or below. At 5 we start getting into the larger predators--dragons, ogre, trolls and the like. If your monster is similar to one of the examples but, due to something extraordinary, simply increase their Might by 1. Tieflings and Aasimar, for example, would be Might 4 as they are natural adventurers from the planes.
Finally, don't feel like you have to answer this question right away. If nothing jumps out at you right away, start by filling in the other details first. Once you're done, take a step back and see how it compares to other creatures. The best advice I can give is to trust your instincts here.