"The next round Vasana decides to cast Demoralize, a 2-point
spirit magic spell, on the same troll. This time she boosts the
spell with 4 magic points, making it effectively a 6-point spell."
This is the closest thing we have to a rule about boosting, and it clearly talks about the size of a spell. Similarly, for dispelling purposes, a Rune Spell is twice as "big" per point as a rune spell, and of course sorcery has the same notion.