OK, so this is drifting into the "Similar Spells" problem, but there does seem to be consensus that FFM should be able to change size, and it should be possible to change the size of Aegis.
How about something along the lines of:
Magnitudes for greater size are a part of the Target parameter, and can be manipulated in the same way as the Target parameter, whether through Virtues, MuVi, or other techniques. However, changing the size does not change the Target parameter.
And added to AoH: Magi may invent versions of this spell with different size modifiers, to cover larger covenants.
Does that look likely to work? (Ignoring the "similar spells" problem, at least for now.)