The larger the state, the more internal resistance there will be. Not everyone in Texas or Florida will want to secede, and everybody can buy a gun. It’s one thing to gather some pride boys and meal team six larpers in the town square and march around for a bit, but it’s an entirely other thing to defend territory when you’re outnumbered, outgunned, and half the population is against you.
Oklahoma and Arkansas might be up for it, but there’s no way secessionists hold Miami or Austin.