Chatted about this a little while ago - https://forum.atlas-games.com/t/confirm-clarification-on-size-ranks-in-spell-effects/6585/1
With an effect as:
Unearthly Magnification of the Beasts
Muto Animal 30. R: Personal, D: Diameter, T: Individual
This spell allows for temporary growth or shrinking of the Magus’ animal form up to Size 11 or down to size -12. The spell may target any creature of up to size +4, but may only change the creatures size by up to 10 size ranks in either direction.
ie. A size a +4, may become up to size +11 or down to -6. A size -3 creature may become size -12 or up to size +7.
(Base 4 for changing creatures size in an unnatural way +/1, +1 to allow for shrinking and growing in the same effect, +1 D:Diameter, +1 mag to affect creatures of up to size +4, +3 to change up to size 11, or down to )