I saw a short story written by someone who writes a Diedne blog and it mentions the mage using magic to increase size. Is this possible and are there any existing spell guidelines for it?
It is certainly Muto Corpus 3, as in Preternatural Growth and Shrinking, and also and the spell Assume the Stature of the Giants of El Cid (MoH p.49, MuCo15). Note thoug that after the initial change the cost to continue to change size is not 1:1.
Many spells change the size of a person or creature, or creatures have their size altered to suit the story needs of an Ars Magica saga. The rationale fro the mechanics below is because +2 mags equates to a 10x times increase in overall mass (as per +2 mags for T: Group), which is demonstrated by the maximum group size being 10x individuals.
Therefore each +1 mag in spell design will add up to 3 size levels (note some spells sampled from the core books seem to use +2 levels instead, but there has been much discussion and the fair middle ground seems to be +3 size per mag).
Each +1 mag in spell design will reduce size by 6 levels (again variable by books, some spells seem to be able to reduce by a far greater amount than increase, I think that is primarily because once a creature is smaller than size -4 they have such low wound levels than it is almost moot).
Each +1 Size = +2 Strength, -1 Quickness, +1 point in the wound level damage ranges (see HoH:MC p. 39).
That means changing a soldier from normal (size +0) size to size +2 adds 4 strength, subtracts 2 quickness, and alters their first “light wound” damage range to 1-7 points instead of 1-5. That might not sound like much of a change but it makes the soldier far more offensively oriented. Slower, harder to kill, and able to inflict more damage.
This is also why reducing an opponent down to -3 or smaller is very effective, it might make them quicker but they have radically reduced wound levels for penalties, which is savage in a prolonged battle.
There is also a point at which the difference between the attacker and defender becomes so extreme that the defender really shouldn’t be able to “parry” to defend. A rationale is: A mouse can avoid the boot of a man easily due to speed, but if the man successfully steps on it, then it really should be dead, or mostly dead.
This is absolutely possible. "Utterly change the appearance or size of a person" is a Muto Corpus level 3 effect (AM5e core, page 132). The sample spell Preternatural Growth and Shrinking on page 131 allows a caster to increase a human target's size by 1 or decrease it by 1 or 2 using that effect.