I've actually rethought my original ruling (mainly because it didn't click that it was a Major Hermetic Virtue). Yes, you can use FFM to modify the target's Size Modifier up or down (if the spell already has at least a +1 Magnitude for Target Size) one magnitude. In addition, you can also modify Magical Senses (p. 113-114) up or down a level (so, you can move a Hearing Intellego spell up to Vision or down to Smell). Because it stands to reason that if you can jigger the size, you can jigger the sense.