I'm somewhat confused by the guidelines for increasing spell difficulty as the size of the target increases. The rulebook says an increase of "one magnitude (five levels)" but one magnitude isn't always 5 levels... exactly.
So if I have a level 3 spell, and raise the size by one which level is the new spell:
a) level 4 (just like increasing one of range, duration, or target)
b) level 8 (literally add 5 levels)
c) level 10 (go up one magnitude)
1, 2, 3, 4, 5, 10, 15, 20... and so on (+5 levels each time)
So an increase of +1 magnitude at level 3 is 4 as it's +1 for size. At level 4 it's 5. At level 5, it's 10.
Low level magic is easy, but once you hit a certain point, it leaps up in difficulty. No magus knows why, and there's a grand mystery for a bonisagus to dedicate their life to.
So, I agree with you and think that that is the by far the most reasonable interpretation of how size guidelines are meant to work. The issue is, when you actually read that section in the book, that is totally not what the book says. I feel like that is actually the least reasonable interpretation of what the core says on how size affects the level of spell. The most likely direct reading of the book is that it always adds 5 levels, making a level 3 spell become of level 8 spell... which frankly seems a bit ridiculous. Thoughts?
Page 114 clarifies under the "Targets" section. "Below level 5, increasing a magnitude only adds one level."
Note that there's nothing specific in the rules that say that spells above Lvl 5 must increase in even increments of +5. Many "General" spells certainly can have Levels of effect that are not even multiples of 5, and the same is technically true with any other spell. Spontaneous effects are often judged as falling between 2 standard Te/Fo Guideline descriptions, and thee is nothing to prevent a formulaic spell from mimicking exactly such an "in-between" effect - within reason and with SG/Troupe approval.
So, if a Wizard had a Lab Total of 14 (or, perhaps more likely, 15 left over and above what they needed to create another spell), they could create a General spell of Level 7 - perhaps a Ward (but not necessarily).
Admittedly, very few SG's and Sagas have (non-general) Formulaic spells that are not measured above Level 5 in even multiples of 5, but it's not unheard of, nor does it break any written rule.
(Level 1-5 are 1st magnitude spells, 6-10 are 2nd, 1-15 are 3rd, and so on.)