How can Catalonia become independent from Spain with a POD post 1800 (except the Spanish civil war)? And how big could it get?
Catalonia specifically, or will Aragon do?
The simplest way would probably be for Spain to fall apart with regionalism during one of the Civil War periods. If Spain survives whole apart from Catalonia it's likely to dedicate the rest of its existence to conquering Catalonia again, which will make life tough for the Catalans.