Hi,
Considering a magus cursed to remain a little child while aging very fast (unaging + age quickly + lesser malediction + dwarf (+ diffcult LR because why not)), I'm pondering the idea to give him the magical focus about "miniature": it would affect objects or persons who are by essential nature, more little than the regular one.
So a bear of size 0 check. A bear size +2 diminished by magic: no.
A house for pupets or dolls. Check. A house transformed and diminished, no.
A great sword: nop. A dagger, nop.
A great sword for a cat sized person, check.
A child? no. A dwarf child: sure.
A human person, no. A dwarf person; sure.
A giant, no. A size 0 giant, sure.
Any mind of intelligent creature? no since they have no size.
Etc.
But I'm not sure: would it be okay? minor? major?
I'd say major because it can encompass many things, but I'd also say minor because it has very few real use except for the magus himself who would be in his focus for corpus spells.
And I could see a world where the focus is not even allowed but it would be a sad world and I'm not going that road if I can avoid it.
The effect of a focus is important. As it will affect many self spells, it is very strong.
There is a lot of utility. The magi can carry around a bunch of specifically crafted mini-items. A muto expert creates a crystal dart spell that instead flings one of the many mini sword nearby which it makes large, etc. Rego expert could do sling of vilano, but it's a mini-sword (dagger size, but with all the trappings of looking like a Great sword).
When making items, there are some where being small is best. A mini chest that expands to full size, and shrinks everything within it when it shrinks, for example.
I would think it's a major focus. It can affect a huge field of Forms and all techniques. Almost any physical item CAN be made easily so it's sized for a small person.... but that does enter into the essential nature issue. It also can affect anyone who is small or frail (at least I have seen a number of characters with Small Frame because of childhood malnutrition). My biggest concern with allowing this sort of definition is how often it will cause a discussion on whether or not it applies! In particular because of the non-living definition... If it applied to only LIVING THINGS that have the Small Frame or Dwarf or similar flaws, that's a flat solid definition that works well.. then a minor virtue argument is strong.
Side note, I also wouldn't really allow it in my own saga, it doesn't really fit with the feel of Magic that I have been cultivating.
added note, I'm not trying to tell you how to play the game, and run with your ideas. My goal is to point out potential problems you might run into and thus can prepare for them.