It’s a hard thing to describe in writing. I’ve read a lot of stories that use magic in their mechanics, and it seems like each is detailed differently.
In the Inheritance Cycle (Eragon) casting a spell that’s too strong can completely use up a person’s energy and kill them. Your body is a gas tank that CAN (and will) run out. The characters get around this issue by storing energy into objects over time and using them to feed their spells (or have dragons contribute the energy). This creates a limit to what the characters can do and helps to build both tension and risk.
In the The Wheel of Time there’s two types of magic. Both sexes weld two sides of the same coin, so to speak, although the male’s side has been corrupted. The more a man (who’s magically adept) uses magic, the more he falls into madness. In this series the magically capable females capture and imprison the male magic users (who are few and far between). They attempt to strip the ability to use magic from them (which usually results in their death). The main character is a male magic user, which as you can imagine makes for an interesting story as he tries to bring an end to the corruption while maintaining a grasp on his slipping sanity.
And let’s look at Star Wars (also books, although having originated on the big screen). Magic is referred to as ‘The Force’ and is used in a multitude of ways (object manipulation, visual projections, mind reading, and even the creation of life to name just a few). In that series a magic user can manipulate the world around them basically by communicating with it (this is the MOST basic interpretation, we won’t get into Midichlorians or the Whills). The stronger a magic user is, the more they can manipulate, only dying from exhaustion in the most extreme cases. although I love Star Wars, the only tension resulting from how the force is handled in these stories is a result of the enemy being stronger in their ability to weld it.
We can also use the Marvel Cinematic Universe as an example (which originated in book form). Removing Asgard as an example (of which magic is just a higher level of technology), we have the magic used by sorcerers (Dr Strange). They pull energy from nearby dimensions and manipulate it as needed. The Ancient One likened it to writing programs on the fly. I’m not sure if there was a limit to how much a magic user could do considering that Marvel had to strip Dr Strange of most powers in the books because he had become unbeatable. This leaves your story with ‘Dragonball Z syndrome’ and means that enemies get stronger and stronger until the reader just gives up because of the ridiculousness taking place. Their limit is simply how much knowledge they have access to.
The reason why I’ve been pondering these examples lately is that I had taken a break from writing The Road of Skulls right at the point that magic is explained in a detailed form. This is the type of concept that seems like it should be easy, but you don’t realize the repercussions it can have later on in your story until it’s too late. Do I set limits on what a magic user can do? What will those limits be? Knowledge, strength, will? What happens if those limits are exceeded?
So, for the next day or so I get to struggle with this concept.