I like that size-changing race idea -- it has some interesting possibilities. For example, they have hidden cities, built up in the tops of normal sized trees, or in small underground caves that are not accessible to normal sized people (i.e. through a tunnel/fissure that is only six inches tall). And if their possessions also change size, that makes for interesting possibilities.
However, I think that such an ability should cost Spell Points (or the equivalent in Power Points -- a set of points used for activating/using inherent magic-like abilities -- such abilities would also require their own skill to use, such would ALWAYS be a Favored Skill for a character with inherent abilities).
For example, the size-changing race - they have 2 default sizes. It costs 3 PP to change size and requires a Willpower-based skill roll to accomplish this (base TN 15, modified by conditions that may disrupt concentration). Or perhaps make the PP cost based on the degree of change. If the default size is 6", then each size increment would be 6" and would cost 1 PP to change (i.e. to become 4' tall, they would have to spend 7 PP, 1 PP for every additional 6" in height). It would cost 11 PP to become 6' in height, and then another 11 PP to shrink back down to 6" tall....
And, yes, this means that they could also assume larger than human forms if they had enough PP....
