To be 100% sure you would need to check the specs for the actual light you intend to install.
My first instinct would be a single gang box mounted sideways. This is only because you say wall sconce. I have a vision of a fixture that shines up which leave little room at the lower part of fixture for wiring so sideways works better.
It could be a round box a single gang mounted vertically or single gang mounted sideways. I have run into all of these instances.
If I knew absolutely nothing about the type of light going in, I may lean towards a single gang mounted vertically.
Typical wall mounted bathroom light fixture uses a round box.
Sorry for the non specific information.