Inventory slots
Each inventory tab starts at 24 slots and can be expanded four slots at a time, up to 128. Expansions are sold by Gold Richie in the Free Market.
The price depends on how many slots the tab already has: the first expansion costs 500,000 mesos, and each one after it costs more.
Stack sizes
Items stack to their original sizes; for example, Arrows for Bow stack to 2,000. Items without a size of their own stack to 100.
Storage
Storage starts with 5 slots and can be expanded up to 30. Gold Richie sells the storage slot expansions too.
When you deposit or withdraw, items fill up any partial stack of the same item before starting a new one.
Account storage
Account storage is how you move items between your own characters. That includes Account Bound items, which cannot be traded or sold.
More characters
The Cash Shop's Extra Character Slot Coupon lets you have up to 9 characters in the world.