The size categories, in order, are: Fine, Diminutive, Tiny, Small, Medium, Large, Huge, Gargantuan, Colossal, and Titanic. Sizes smaller than Tiny are all counted as 'Tiny' for the games rules, and sizes of Titanic+ are all counted as merely Titanic. The difference between being 6 inches tall versus 4 inches tall are not significant for game effects, and the difference between being 100 feet tall and 200 feet tall are the same.
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.