Bootable USB without an OS shows as Windows in Startup Manager
Obviously something went wrong in making the bootable USB stick.
It should be 8GB.
Why not use a little tool that does it for you:
Diskmaker_X,  see http://diskmakerx.com
You have to have the OSX installer on your disk: if you already installed it and it is gone, download it again from the Purchased tab of AppStore.