Mirrored from Sythyry.
Relee asks: I don’t know much about World Tree magic. How do you folk measure the power of a spell to determine how strong it is? Now I’m curious what your most powerful spell is, and what the most powerful spell you’ve heard of is, for comparison!
A typical spell, when it is cast, has an appearance of power about it, much as a flame has a brilliance or luminosity. Different casts of the same spell vary in power — even for the same person, they vary somewhat. We measure the power of spells by the intensity of their appearance.
Often, the measured power is correlated with something measurable in other ways. A simple spell of power  makes [one yard] of rope-vine; a spell of power  makes [two yards]. I typically make about twenty-some yards with that spell, with a typical power [of 100 or a bit more].
[I translate spell powers as well as distances into English; Sythyry uses different units for both. -bb]
There is a separate dimension, of spell complexity. More intricate spells (which generally do more, or do more subtle or harder things) are more complex. The rope-vine-making spell is the simplest quantum, called 5 (because it takes 5 cley to graft that spell on — no spell takes fewer, and all spells take multiples of 5, hence they are proper quanta.) A good professional mage has a couple of complexity-20 spells in her specialty. The best healers have a couple of 30′s. My most complex spell, Dancing in the Garden of Statues, has complexity 100; I do not have many that complex, or even close. (Vae can improvise spells of complexity 80 on nearly any topic, and power [150-200], without any effort at all; this makes her a truely fearsome creature indeed. My best spell is better than her efforts in that topic — which is impressive indeed! — but she is nearly as good as my best in everything. And I have very limited cley, and she has no limits whatever. )
[There is no need to translate spell complexities, since those are simply numbers that have a simple physical explanation.]
Spell effects are often exponential in the complexity. A complexity-5 spell can make a few yards of rope-vine. A complexity-10 variant can make the same number of tens of yards of rope-vine — and a mage who has both variants grafted and can cast both, will cast them at precisely the same power [or, more accurately, at the same distribution of possible powers. -bb] A complexity-15 spell will make so many hundreds of yards; a complexity-20 spell so many thousands of yards. The rate decreases after that, typically, so a complexity-30 (rather than 25) spell is required to make so many myriads of yards, and a complexity-45 spell so many tens of myriads.
For extra confusion, not all topics behave this way. Attack spells increase very slowly; a complexity-25 spell does only slightly more damage than a related complexity-5 spell.
Of course, high-complexity spells are hard to learn, hard to cast, hard to invent, and hard to box; they are tremendously expensive, and very few people can actually cast them.
The power of one’s spells is only a mediocre measure of how good a mage one is. Two mages might be able to achieve power  on the average, say, but if one only has complexity-5 spells and the other has several complexity-30′s, the second will be far more effective with her magic. [Also, a mage who averages power 40 in a Noun+Verb combination will probably be able to cast spells of complexity 30 or so, but probably not more. Sythyry thinks this is too obvious to need mentioning, but zie is wrong. -bb] However, a bit of money — well, a lot of money — and a few months’ work could give the first mage all the second one’s spells, and make the two be roughly equal.
Anyhow, it is easy to measure spell power, and spell power is strongly correlated with everything else that matters about a mage — except for a number of important disciplines, but never mind that — so we measure by power as a convenient shorthand.
There is no power level at which one is given a title of advanced magic, like “sorcerer” (meaning “very impressive spell user”) or “wizard” (meaning “even more so”). These titles are awarded informally: if enough sorcerers and wizards call you a sorcerer or wizard, you can call yourself one too and nobody will sneer. Usually one must impress the people of the rank one aspires to, in some way. I did it with a very clever time-distortion enchantment, plus surviving a century of nendrai wrangling (and, more to the point, breaking many of her curses despite having nothing like the necessary power or complexity of my own spells.)
There are of course many further elaborations and important details not mentioned herein, but I daresay I may have already melted your ear if not your brain, so I will shut up now.
[They can be found in the World Tree sourcebook. Rather to my surprise, I've only deviated from the sourcebook in a few ways in a decade of Sythyryzing. E.g., the attitudes towards transaffection are moderately different in Sythyry's world than in the sourcebook, and the rules on making other people immortal seem to be rather harsher for Sythyry. The sourcebook is lots of fun to read, and only half of it is game rules! Buy it, read it, and see where else you can catch me contradicting it! -bb]