First thing that hit my mind when I read this was Fiona going "F^@k yeah!"
But I agree, having to take a size modifier for spells that you cast on yourself is counter-intuitive. After all, if you spend a season inventing the spell, you would have taken the fact that you're half Giant into account.
As far as Spontaneous magic goes, I'm torn but leaning toward ending the size modifier (by about two degrees). On the one hand, my above argument would apply. On the other hand, everything you're taught about Corpus magic says that it only works up to a point, and that you spend a season inventing a spell (e.g. Fiona's Aura of Ennobled Presence) to work around that core concept.
So, I'm thinking Yes on Personal Formulaic Spells, and Maybe on Personal Spontaneous Spells.