There are no standard resolutions available. The optimal resolution varies between headsets depending on display panel resolution and lens distortion correction. The VR runtime provides a recommended resolution, which is what the engine applies to the VR view by default.
With practically every PCVR runtime released, users can already adjust the recommended resolution from the runtime settings. It’s common for games to have an additional resolution scaling slider in-game as well. Unreal only reads the recommended resolution at startup, so it might be a good idea to have an in-game setting to allow adjusting it on the fly. The resolution from the runtime is scaled by the vr.PixelDensity float console variable with the default value 1.0, so it’s possible to have a continuous slider that adjust the resolution up and down.