Does the Monstrous Combat Options rule of breath weapons scaling with creature size apply to PC's?

Question

While discussing build options with a fellow party member (an Alchemist), they mentioned a rule granting breath weapons increased area when the user increases size category. After a bit of searching, I was able to find the rule in question:

Breath Weapon: The monster gains a breath weapon that deals 1d6 points of damage + 1d6 per CR. A target can attempt a Reflex saving throw to take half damage. If the breath weapon is a cone, it’s 30 feet long, increasing by 10 feet for each size category above Medium, and decreasing by 5 feet for every size category below Medium. If the breath weapon is a line, its area of effect is twice as long as a cone would be.

However, said rule was located in a section about monster creation/advancement, and now we're debating whether or not it applies to PC's changing size due to spells or buffs. Is there any other RAW indication whether or not gaining or losing a size category would generally affect breath weapon areas in this manner, or would this ultimately be a matter of GM fiat?

Answer 1

That is an option for monster creation only. It would apply to PCs only in the case that a GM allowed his players to create monsters of up to CR X to use as characters.

The size of the "default" breath weapon in the monster creation rules scales to the monster's size, but also wouldn't change if they were resized during play - for example, a reduce spell on a giant who had a breath weapon wouldn't make that breath weapon smaller.

Unless either the source of the breath weapon or the source of the size specify otherwise, there are no rules present in PF1 that cause recalculation of the area of special attacks due to size changing (except special attacks that rely on your melee reach, and auras and emanations that radiate from the user).