Each Target level includes all of the possible sizes of that target, even though the spell level changes. The size of the target can be manipulated in the same way as the Target parameter, whether through Virtues, MuVi, or other techniques, but these are all changes within the same type of Target. For example, a Target Individual spell with +2 magnitudes for size, to affect an Individual 100 times the normal size, has the same Target parameter as a Target Individual spell with no magnitudes added for size.
Just a minor thing... if you already took Ceremonial Casting for Mastery, invest in Mastery 2, and take Penetration, it takes 1 or 2 seasons to gain a further +3 to penetration. (+1 from the bonus for the casting bonus, and +2 from the mastery).
You have Mastery 1 (Ceremonial Casting), to get another +3, you practice, or use Adventure XP, to get Mastery 2 (Ceremonial Casting, Penetration).
Since you only gained 1 point of Mastery, the casting bonus increases by +1, but you get an additional +2 to Penetration, because the Penetration Mastery adds the total Mastery score to the Penetration bonus. Hopefully, your casting bonus was already high enough to successfully cast the spell, so the +1 to casting bonus from raising Mastery, also adds to the Penetration bonus, for a grand total of +3 Penetration.
Depending on the spell, every little bit of Penetration might matter. Things designed to be used against high MR, such as Might Wards and Might Strippers, are often limited in the maximum level that they can be learned and used effectively to far lower than the maximum that could be cast. Typically that is about half the highest that could be cast.
Some combination of Mastery can improve that. Ceremonial Casting and Penetration can let you successfully cast a more powerful version, while Multi Casting could let you use a weaker (but high Pen) Might Stripper to do a "one round kill" on a being that you could not affect with a more powerful version.
Being able to do all of that fairly cheaply is one of the advantages of FM.