Large and Little Spellcasting.

How much fatigue does a pixie spend to charm a human? To zap him with lightning? To turn him into a toad? What about a giant? What if it is the giant who's casting the spells?

If your campaign is going to have pixies and giants as well as humans, all slinging spells, you'll need to give this some thought.

Speaking of size and gaming, if you haven't read T-bone's GULLIVER yet, stop reading and go here.

The Value of Fatigue

Fatigue is the energy used to power spellcasting. In the following, it will be assumed that fatigue is based on HT. Thus, the number of fatigue points a given creature has will not be linked to size.
So, how much magic can a giant or micro work? There are a few options:

Fatigue is fatigue. A point of fatigue will work the same magic, whether it's from a pixie or a giant.
Fatigue scales with linear dimension, making a point of giant fatigue worth much more for spellcasting purposes.
Fatigue scales with linear dimension, and so does granularity. This is my favourite option.

If option 1 is in place, everybody can cast the same spells, provided they know them, at the same fatigue cost. Micro sorcerers blast their (micro-sized) foes out of existence easily. Giants can cast spells at each other until they pass out, but it won't matter much. I don't like this option.

If option 2 is in place, every spellcaster has a Fatigue Value, FV. FV is proportional with his linear size: Some may use ST/10, others just size category multiplier. For every (FV) fatigue points the spell requires, it costs one point of fatigue.

If option 3 is used, every spell costs fatigue as usual, but the effects are on the caster's own scale.
What does this mean?

For damaging spells, the damage is from the casters point of view. A 1d flame jet cast by a man will annoy a giant, harm a man and incinerate a micro. The same flame jet by a micro will harm a micro, annoy a man, and fail to grab the attention of a giant. If cast by the giant, the human will die almost as messily as the micro.
For information spells, range is proportional to the caster's scale.
For spells that affect an area, including any that affect a body, the number of hexes the caster must pay for is determined by the targets size as perceived by the caster. A human is a multi-hex creature to a micro, while a human could affect multiple micros with a 1-hex spell. Note that this includes Regular spells.
Range penalty is modified by the caster's size; 3 yards is nothing to a giant but a lot to a micro.
For spells affecting units not measured in hexes, multiply mass limits by scale^3, areas by scale^2.

Note that the minimum cost for a spell is for one hex, relative to the caster!

The Size of a Soul

How much energy does it take to affect a mind or a soul? Each of these could be:

A fixed "size". This makes the soul or mind of a larger opponent cheaper to affect than his body. It also makes mind control hideously expensive if you're a small spellcaster.
Equal in size to the body. If your opponent is triple-sized, any mind- or soul-affecting spells will cost triple as well.
What's this nonsense about Soul Size? A charm spell is a charm spell, never mind the size of caster and victim.

You can easily use one system for the mind, another for soul, if you want to.
Which system should you use?
If you don't mind powerful mages controlling huge beasts, or small pixies charming the PCs, use system 2. If you dont want this to happen, use system 3.
If your cosmology requires it, system 1.

Points

I recommend keeping points out of this - it will be rather complicated. Call it a genre/campaign rule.