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By default, when starting, the mixer takes the video size of the 1st source, if already running. If no source is running it takes 320×240 by default.
To set the video size independently of the sources, invoke UseNearestVideoSize (w, h, true) before starting the mixer, e.g.:
VideoGrabberMixer.VideoSource = vs_Mixer
VideoGrabberMixer.UseNearestVideoSize (1280, 720, true);
VideoGrabberMixer.Mixer_AddToMixer (VideoGrabberSource1.UniqueId, …)
VideoGrabberMixer.StartPreview();