The size categories, in order, are: Fine, Diminutive, Tiny, Small, Medium, Large, Huge, Gargantuan, Colossal, and Titanic.
For every step you move down in size, you gain:
Sized-small player characters gain these bonuses/penalties because they are down a step from size medium.
For every step you move up in size, you gain:
All adjustments are considered 'Size' modifiers. These adjustments would apply to any creature increasing their size, regardless of their 'base' size.