There is no property/method to set the (video)resolution for QML MediaRecorder. However, its C++ counterpart, i.e., QMediaRecorder, does have a method QMediaRecorder::setVideoResolution. However, that method is not Q_INVOKABLE. Nor there is any Q_PROPERTY related to resolution.
Also I can see in QML Camera example, that the lines about 'resolution' is commented out. As if the functionality of setting resolution was (or will be?) available. Anyway, it is not available now as of 6.4 and 6.5.