Mixer: how I can set the mixer video output size? my cameras are configured as 1280×720, but the output file is 320×240.

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();