The loudest Bluetooth speakers are the biggest ones. Sometimes they're referred to as boom boxes or "party" speakers. Some of those speakers are portable and have built-in rechargeable batteries while the biggest party speakers literally have wheels on them for transport and need to be plugged in with an AC adapter because they require so much power.  We have a list of best wireless boom box speakers if you're looking for a bigger Bluetooth speaker. It's also worth noting that Bluetooth speakers from the same company can be daisy-chained together to augment their sound or paired together as a left-right stereo pair. Often, the best way to get bigger sound is to pair two Bluetooth speakers together in a stereo pair.