But which benefits derive from the ACTUAL size you choose and which from the spell?
None of the listed benefits depend on the actual size of dragon you choose. There are a few things that depend on the actual choice of dragon type though:
Form of the Dragon
You gain […] a breath weapon, and resistance
to one element. […] Your breath weapon and resistance depend on the
type of dragon. […] In addition, some of the dragon types grant
additional abilities, as noted below.
However, nothing depends on the chosen size, a Medium dragon gets the same benefits as a Huge dragon from Form of the Dragon III.
The "size" of the bonus is just the name of the bonus type (like enhancement bonus, competence bonus, dodge bonus, …), and is unrelated to the "size" of the creature.
This confusion comes up frequently, due to the fact that the bonus type "size" unfortunately uses the same word as the physical space rule element "size". Whenever you are unsure about these things, just mentally rename every "size bonus" into an "awesomeness bonus" (or something like that).
do I get everything stated in Form Of The Dragon 3 (size bonus to ability scores, +8 natural armor bonus, blindsense 60 feet, darkvision 120 feet, damage reduction 10/magic, etc...) except for the size and consequently reach on attacks?
So yes, this is the right answer, you get everything from Form of the Dragon III (abilities, damage with your natural weapons and breath attacks, …), except for things that rely on a creature's actual size like natural reach, CMB & CMD modifiers, attack & AC modifiers, fly & stealth modifiers, and so forth.