Giant Blood, size modifying spells and standard Corpus spells

I have gotten myself confused again, regarding Size limits.

IMS is a Giant Blooded Magus has has invented a version "Preternatural Growth and Shrinking" that affects a a Size +2 Individual (with a Personal range) - ie himself.

If the Giant Blood shrinks himself to Size +1 or less, I assume that various standard Corpus spells can now affect this shrunken Giant. Will these standard Corpus spells lose their effect on the Giant Blooded when he reverts back to normal +2 size?

Take for instance the Moon duration spell "Purification of the Festering Wounds". If the Giant Blooded mage recasts his personal version of "Preternatural Growth and Shrinking" immediately after Dawm and Dusk for the entire Moon Duration, to revert to Size +1, does he get the full benefit from the single casting of "Purification of the Festering Wounds"?

I would follow this rule from shape shifting in HoH:MC p.22, as it's the closest we have:

any ongoing magics using these Forms cast prior to transformation remain in effect.

If this is true of effects with a change from human to animal that usually changes Size as well, I don't see any reason changing only Size wouldn't work the same way.