Hey, TJ. Welcome to planDisney.
You’ve asked a great question! "Nice work, pal." And it’s the balancing act we all know too well: bringing a bag that’s big enough to hold ponchos, snacks, backup snacks, and five pairs of Mickey ears… but small enough to not end up wrestling it into a Dumbo elephant. Totally get it.
The size limit you mentioned (24" x 15" x 18") applies to bags allowed into the parks, not necessarily to what’s allowed on individual attractions. Thankfully, most rides at Walt Disney World are super bag-friendly. On attractions like Pirates of the Caribbean, Haunted Mansion, Seven Dwarfs Mine Train, and many others, you can keep your bag with you, either at your feet or in a small pouch in front of you.
There are a couple of exceptions to know about.
TRON Lightcycle / Run requires all bags to be stored in complimentary lockers before you board (they’re easy to use—just scan your MagicBand or ticket to lock and unlock).
Kali River Rapids has lockers nearby too, and while you can bring your bag on the raft, be prepared: it’s not called a “rapids” ride for nothing. If you keep your bag with you, waterproof it! As for
Soarin’ Around the World, there’s storage under your seat, but if your bag doesn’t quite fit, a friendly Cast Member will help find a safe spot for it nearby during the flight.
I hope that's helpful, TJ!
Mike