OBS isn't a converter, it makes it's own video files. The file size and quality depends on how you've set it up.
You can do screen capture with it that would work for you. I remember re-doing one of my FSO films that way when I'd forgotten to start recording video in OBS but recorded in ahfilm. I ran the film in the viewer in full screen, recorded that with OBS. Had to use the hat switch to get views as I run TIR. But that way will work. OBS will create an mp4 video of the screen capture.
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