You've got a couple of problems with your spec and will need to work around them to proceed.
First, a USB connected speaker would need its own audio adapter onboard, or drivers that interface with the audio adapter on the host machine and a board in the speaker to convert the digital signal to an analog form internally. Outside of headphones, there's not a lot of demand for that, so you may be searching for a product that simply isn't.
Second, the output of a speaker is limited by the amount of power which it can receive. The voltage available is only 5 volts, and in all forms the maximum current is less than a full amp (.5 for most, .9 for USB 3.x). Depending on the USB port, that gives you under 5 Watts to work with. At max volume, you're looking at 93-96 decibels, but keep in mind that the above is peak for the USB port, and some of that power will go to converting the digital signal to analog. This means your actual results will probably be closer to 75-80db, which gets you not much louder than a conversation. Speaker size also matters, and this will inhibit the max volume the speaker is physically capable of putting out there. A speaker that is under 2 inches is going to output so little sound under the power a USB port can give it that your audio will require a quiet room or your head close to it to hear it.
Small electronics usually wire an analog speaker directly into an onboard component, and if you disassemble something like an old Tiger handheld game, you'll find that tiny sound was produced by speakers that still took up a large chunk of the case. The way headset designers make it work is through additional components you don't have available that increase the size of their solution. They also have the benefit of the speaker being so close to your ear that the overall output power is less important. USB sound cards do something similar, and you'll find that all external sound cards that produce powerful, non-headset sound use a secondary power supply plugged into the wall.
My recommendation would be to use a Bluetooth adapter in the USB port, and use a battery-powered rechargeable portable speaker. Those you can find all day long, resolve your audio quality and power problem, and your analog audio jack will remain open. It's been awhile since I worked with a Pi so I don't know how hard it is to get a BT stack set up on one these days, but it's a wheel that already exists.