You'll want to start by translating the text.
In English, it reads: "Measure what is measurable, and make measurable what is not so."
This text is telling you what to do with the differently sized letters.
Analyze each line separately (except the 3rd line, which has no oddly sized letters).
Final Solution: If you arrange the too-large letters from largest to smallest, the two lines read:
Well, it must be an alphabet, but it's not a language I recognize...
...at least not a language of this planet.
The moon will illuminate your path.
Search for a few moon-themed scripts.
Final Solution: This is the "Moon" alphabet, developed by William Moon. The plaintext reads: "The Ourorobos has a mole in the CIA, he goes by the codename Manticore."
Well, Sole is Italian for "Sun..."
Eight lines... and eight planets.
But they may not be the planets' names in English.
If 1 is the first letter in the solution, then the first letter is A.
Final Solution: The planets, in Italian, are:
Taking the letters marked by numbers and ordering them numerically gives us: AUSTIN.
Just like the other stamps, this will be more useful later.
They are also solved in a similar way.
What are these words in Italian?
Final Solution: "Pear" and "Brooms" in Italian become "Pera" and "Scope" which, combined, become "Periscope."