On a separate question, here, it is agreed that casting giant form 2 on yourself, and then having legendary proportions applied stack, but my question is, what is the total of the buffs? I know size bonuses don't stack, so do you get the +8 to strength from giant form 2, or because you are now gargantuan, do you get a larger bonus? And does this logic follow through on the dexterity penalty, constitution bonus, and so on?
Those two spells combined don't increase your size to Gargantuan
You can only be affected by one polymorph spell at a time. If a new polymorph spell is cast on you (or you activate a polymorph effect, such as wild shape), you can decide whether or not to allow it to affect you, taking the place of the old spell. In addition, other spells that change your size have no effect on you while you are under the effects of a polymorph spell.
Also, according to this FAQ item, size changes never stack, but this isn't talking about size bonuses, but size changes:
As per the rules on size changes, size changes do not stack, so if you have multiple size changing effects (for instance an effect that increases your size by one step and another that increases your size by two steps), only the largest applies.
Which means that if one spell increases your size by one step, and another increases it by two steps, you will only increase your size by two steps, never by three. If one spell increases your size from Medium to Huge (like Giant Form II), and another increases your size by one step (like Enlarge Person), your effective size will remain Huge, as that is a largest size changing effect.
There is effectively little difference in the wording on how your size increases between Enlarge Person and Legendary Proportions, so even though all penalties will stack, some of the bonuses will not.
So, the final result will be:
- Size: Huge (with all penalties and bonuses to carrying capacity and skill checks)
- +8 size bonus to Strength
- -2 penalty to Dexterity
- +6 size bonus to Constitution
- +6 natural armor bonus
- low-light vision
- +10 foot enhancement bonus to your speed
- DR 10/adamantine
Plus the extra abilities of the giant form assumed, as explained under Giant Form II. So the only real benefit of casting both spells on yourself is the extra Damage Reduction.